lEx  ICtbrtB 


SEYMOUR  DURST 


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vm>-«"i )  i)     t.  "    t4mm  vg..t  ^   _____  


The  Catskill  and  Croton  water  supply  systems  of  New  York  city 


WITH  THE  AUTHORS  COMPLIMENTS. 


Education  Department  Bulletin 

Published  fortnightly  by  the  University  of  the  State  of  New  York 

Entered  as  second-class  matter  June  24,  1908,  at  the  Post  Office  at  Albany,  H.  Y.,  under 
the  act  of  July  16,  1894 


No.  489 


ALBANY,  N.  Y. 


February  15,  191 1 


New  York  State  Museum 

John  M.  Clarke,  Director 
Museum  Bulletin  146 

GEOLOGY  OF  THE  NEW  YORK  CITY 
(CATSKILL)  AQUEDUCT 

STUDIES  IN  APPLIED  GEOLOGY  COVERING    PROBLEMS  ENCOUNTERED 
IN  EXPLORATIONS  ALONG  THE  LINE  OF  THE  AQUEDUCT  FROM 
THE  CATSKILL  MOUNTAINS  TO  NEW  YORK  CITY 

BY 

CHARLES  P.  BERKEY 


PAGE 

Introduction     and  acknowledg- 
ment   5 

I  General  features   9 


Ch.    1  Catskill     water  supply 

project    9 

2  Problems  encountered  in 
the  project   17 

3  Relative  values  of  differ- 
ent sources  of  informa- 
tion and  stages  of  devel- 
opment   25 

4  General  geology  of  the 
region   29 

II  Geologic     problems     of  the 
aqueduct   75 

Introduction   75 

Ch.    1  General  position  of  aque- 
duct line   77 

2  Hudson  river  canyon ....  81 

3  Geological  conditions 
affecting  the  Hudson 
river  crossing   97 

4  Geological  features  in- 
volved in  selection  of  site 

for  the  Ashokan  dam.  ...  109 

5  Character  and  quality  of 
the  bluestone  for  struc- 
tural purposes   1J7 


PAGE 

Ch.    6  The  Rondout  valley  sec- 
tion  125 

7  The  Wallkffl  Valley  sec- 
tion  149 

8  Ancient  Moodna  Valley. .  153 

9  Rock  condition  of  Foundry- 
brook   163 

10  Geology  of  Sprout  brook.  171 

11  Structure  of  Peekskill 
creek  valley   175 

12  Croton  lake  crossing   183 

13  Geology  of  the  Kensico 
dam  site   191 

14  Stone  of  the  Kensico 
quarries   195 

15  The  Bryn  Mawr  siphon..  201 

16  A  study  of  shaft  13  and 
vicinity  on  the  New  Cro- 
ton aqueduct   209 

17  Geological  conditions 
affecting  the  location  of 
delivery  conduits  in  New 
York  city   215 

18  Areal  and  structural 
geology  south  of  59th 
street   231 

19  Special  exploration  zones.  237 

20  The  general  question  of 
postglacial  faulting   271 

Index   277 


ALBANY 

UNIVERSITY  OF  THE  STATE  OF  NEW  YORK 
191  I 

M232r-Apio-2.soo 


ru>.  1^ 

STATE  OF  NEW  YORK 
EDUCATION  DEPARTMENT 

Regents  of  the  University 
With  years  when  terms  expire 

^  13  Whitelaw  Reid  M.A.  LL.D.  D.C.L.  Chancellor  New  York 

917  St  Clair  McKelway  M.A.  LL.D.  Vice  Chancellor  Brooklyn 

919  Daniel  Beach  Ph.D.  LL.D.      -   -    -    -    -  Watkins 

914  Pliny  T.  Sexton  LL.B.  LL.D.  -----  Palmyra 
912  T.  Guilford  Smith  M.A.  C.E.  LL.D.    -    -    -  Buffalo 

918  William  Nottingham  M.A.  Ph.D.  LL.D.  -    -  Syracuse 
922  Chester  S.  Lord  M.A.  LL.D.     -----  New  York 

915  Albert  Vander  Veer  M.D.  M.A.  Ph.D.  LL.D.  Albany 
911  Edward  Lauterbach  M.A.  LL.D.  -   -   -   -  New  York 

920  Eugene  A.  Philbin  LL.B.  LL.D.    -    -    -    -  New  York 

916  Lucian  L.  Shedden  LL.B.  LL.D.    -    -    -    -  Plattsburg 

921  Francis  M.  Carpenter   -------  Mount  Kisco 

Commissioner  of  Education 

Andrew  S.  Draper  LL.B.  LL.D. 

Assistant  Commissioners 

Augustus  S.  Downing  M.A.  Pd.D.  LL.D.  First  Assistant 
Charles  F.  Wheelock  B.S.  LL.D.  Second  Assistant 
Thomas  E.  Finegan  M.A.  Pd.D.  Third  Assistant 

Director  of  State  Library 

James  I.  Wyer,  Jr,  M.L.S. 

Director  of  Science  and  State  Museum 

John  M.  Clarke  Ph.D.  D.Sc.  LL.D. 

Chiefs  of  Divisions 

Administration,  George  M.  Wiley  M.A. 
Attendance,  James  D.  Sullivan 

Educational  Extension,  William  R.  Eastman  MA  M.L.S. 

Examinations,  Harlan  H.  Horner  B.A. 

Inspections,  Frank  H.  Wood  M.A. 

Law,  Frank  B.  Gilbert  B.A. 

School  Libraries,  Charles  E:  Fitch  L.H.D. 

Statistics,  Hiram  C.  Case 

Trades  Schools,  Arthur  D.  Dean  B.S. 

Visual  Instruction,  Alfred  W.  Abrams  Ph.B. 


New  York  Stale  Education  Department 

Science  Division,  April  6,  1910 

Hon.  Andrew  S.  Draper  LL.D. 

Commissioner  of  Education 

Sir:  The  extraordinary  engineering  operations  which  have  been 
undertaken  in  the  effort  to  provide  the  city  of  New  York  with  an 
adequate  waUr  supply  have  illuminated  in  most  unexpected  manner 
the  geological  .structure  and  history  of  the  region  of  the  Hudson 
valley  south  of  the  Catskill  mountains.  So  broad  has  been  the 
scientific  scope  of  this  engineering  problem  and  so  direct  its  de- 
pendence on  geological  structure  that  the  Commissioners  of  the 
New  York  City  Board  of  Water  Supply  early  found  it  of  essential 
moment  to  enlist  in  their  service  a  corps  of  trained  geologists. 

In  1909  an  agreement  was  effected  between  the  Board  of  Water 
Supply  and  the  State  Geologist,  in  pursuance  of  which  the  geolog- 
ical data  acquired  in  the  preliminary  and  final  surveys  for  the  aque- 
duct were  intrusted  to  Dr  Charles  P.  Berkey,  a  member  of  the  staff 
of  the  board  as  well  as  of  the  geological  survey,  for  summation  and 
presentation  of  their  hroader  and  more  important  bearings. 

I  transmit  to  you  herewith  Dr  Berkey's  report  thereupon,  entitled 
Geology  of  the  New  York  City  (Catskill)  Aqueduct.  It  is  a 
document  of  high  value  not  only  in  enlarging  and  perfecting  our 
knowledge  of  the  geological  structure  of  the  commercial  center  of 
the  United  States,  but  its  data  and  conclusions  must  prove  of  pro- 
found importance  to  all  large  engineering  and  architectural  propo- 
sitions concerned  with  the  region  of  the  lower  Hudson  valley. 

[3] 


4 


NEW  YORK  STATE  MUSEUM 


I  therefore  submit  this,  subject  to  your  approval,  for  immediate 
publication  as  a  bulletin  of  the  State  Museum. 

Very  respectfully 

John  M.  Clarke 

Director 

State  of  New  York 
Education  Department 

commissioner's  room 

Approved  for  publication  this  Jth  day  of  April  1910 


Education  Department  Bulletin 

Published  fortnightly  by  the  University  of  the  State  of  New  York 

Entered  as  second-class  matter  June  24,  1908,  at  the  Post  Office  at  Albany,   N.  Y., 
under  the  act  of  July  16,  1894 

No.  489  ALBANY,  N.  Y.  February  15,  19 1 1 


New  York  State  Museum 

John  M.  Clarke,  Director 
Museum  BuUetin  146 

GEOLOGY  OF  THE  NEW  YORK  CITY  (CATSKILL) 

AQUEDUCT 

STUDIES  IN  APPLIED  GEOLOGY  COVERING  PROBLEMS  ENCOUNTERED  IN 
EXPLORATIONS  ALONG  THE  LINE  OF  THE  AQUEDUCT  FROM  THE 
CATSKILL  MOUNTAINS  TO  NEW  YORK  CITY 

BY 

CHARLES  P.  BERKEY 

INTRODUCTION  AND  ACKNOWLEDGMENT 

It  is  the  writer's  hope  that  the  series  of  studies  brought  together 
in  this  bulletin  may  help  to  effect  a  wider  appreciation  of  the  prac- 
tical usefulness  of  geology.  The  volume  contains  a  summary  of 
the  local  geologic  facts  and  the  general  principles  found  helpful  in 
solving  some  of  the  problems  encountered  in  a  single  great  engineer- 
ing enterprise.  The  summary  is  accompanied  by  brief  discussions 
of  the  methods  employed  and  of  the  final  results  or  conclusions 
reached.    It  is  therefore  essentially  a  study  in  applied  geology. 

Seldom  has  so  favorable  an  opportunity  been  afforded  to  follow 
extensive  exploratory  work  and  check  geologic  hypothesis  or  theory 
by  subsequent  proof.  And  still  more  seldom  have  engineers  in 
charge  of  similar  works  so  fully  appreciated  the  value  of  geologic 
investigations  and  the  extent  to  which  they  can  be  utilized  as  a 
guide. 

More  credit  is  due  to  Mr  J.  Waldo  Smith,  chief  engineer  of  the 
Board  of  Water  Supply  of  the  City  of  New  York,  than  to  any  one 
else  for  appreciating  the  importance  of  the  geologic  complexity  of 

[5] 


NEW  YORK  STATE  MUSEUM 


the  Catskill  Aqueduct  problem.  His  exceptional  insight  into  its 
nature  led  to  the  adoption  of  measures  in  this  direction  that  are 
now  proved  to  have  been  fully  justified.  A  staff  of  geologists  has 
been  maintained.  From  time  to  time  engineers  of  the  regular  staff 
who  have  shown  unusual  aptitude  in  such  investigations  have  been 
assigned  to  special  duty  on  geologic  exploratory  work.  In  the  pre- 
liminary investigations  of  the  Northern  Aqueduct,  Division  Engineer 
James  F.  Sanborn  was  very  intimately  connected  with  the  geologic 
work.  With  him  the  writer  worked  out  many  field  studies  that 
later  formed  the  basis  of  advisory  reports,  covering  locations,  kinds 
of  explorations  to  be  made,  and  interpretations  of  data.  No  one 
has  had  a  better  grasp  of  both  the  geologic  and  the  engineering 
aspects  than  Mr  Sanborn.  It  is  with  great  pleasure  that  the  writer 
acknowledges  many  valuable  suggestions  and  much  help  through 
association  with  him.  In  the  later  exploratory  work  within  the  city 
similar  service  has  been  rendered  by  Mr  John  R.  Healey,  who  has 
much  to  do  with  the  geologic  detail  of  the  delivery  conduit  data. 
The  consulting  geologists  employed  by  the  board  were  Professors 
James  F.  Kemp,  W.  O.  Crosby  and  the  writer. 

A  special  debt  is  acknowledged  to  Prof.  James  F.  Kemp,  consult- 
ing geologist  of  the  board,  whose  confidence  in  the  writer's  work 
originally  brought  him  into  touch  with  these  investigations  as  an 
assistant,  and  with  whom  since  that  time  many  joint  reports  to  the 
board  have  been  written. 

Valuable  advice  and  assistance  in  arranging  for  the  issue  of  this 
report  has  been  given  by  Department  Engineer  Alfred  D.  Flinn  of 
Headquarters  Department.  For  some  of  the  corrections  and  sug- 
gestions special  acknowledgment  is  made  to  Department  Engineer 
Thaddeus  Merrimar  . 

The  department  engineers,  Robert  Ridgway  of  the  Northern 
Aqueduct,  Carlton  E.  Davis  of  the  Reservoir,  Merritt  H.  Smith,  for- 
merly of  the  Southern  Aqueduct,  Frank  E.  Winsor  of  the  Southern 
Aqueduct,  William  W.  Brush  and  Walter  E.  Spear  of  the  City  De- 
livery have  given  every  facility  for  gathering  geologic  data  within 
their  territory  and  have  contributed  largely  to  the  better  understand- 
ing of  their  special  fields. 

The  geologic  matter  relating  to  special  problems  has  been  worked 
out  with  the  aid  of  the  division  engineers  in  direct  charge  in  the 
field.  Among  these  must  be  mentioned  L.  White  of  the  Esopua 
division,  William  E.  Swift  of  the  Hudson  river  division,  A.  A. 
Sproul  of  the  Peekskill  division,  Lawrence  C.  Brink  of  the  Wall- 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


7 


kill  division,  J.  S.  Langthorn  of  the  Ashokan  reservoir,  Wilson 
Fitch  Smith  of  the  Kensico  division,  T.  C.  Atwood  of  the  New 
York  city  delivery  division. 

The  data  included  in  the  tabulation  of  this  bulletin  have  been 
gathered  largely  by  others.  Many  of  the  explanations  and  conclu- 
sions are  the  outgrowth  of  the  work  of  engineer  and  geologist, 
together.  A  large  number  of  associates  are  engaged  on  this  public 
work  in  such  relations  to  one  another  that  the  individuality  of  each 
is  obscured  in  the  common  effort  to  reach  an  enviable  efficiency  and 
success  for  the  whole  enterprise. 

The  combined  efforts  of  many,  unselfishly  given,  have  thus 
brought  together  a  total  far  in  excess  of  what  any  one  individual 
could  accomplish.  Acknowledgments  "should  therefore  be  made  to 
those  members  of  the  staff  of  the  Board  of  Water  Supply  who  can 
not  in  the  nature  of  the  case  be  mentioned  by  name.  Were  it  not 
for  their  cooperation  the  great  mass  of  data  here  summarized  could 
not  have  been  compiled. 

Charles  P.  Berkey 
Special  Geologist,  New  York  State  Geological  Survey; 
Consulting  Geologist  New  York  City  Board  of  Water  Supply 
Columbia  University,  New  York  City  November  i,  iqio 


I 


GENERAL  FEATURES 

CHAPTER  I 
CATSKILL  WATER  SUPPLY  PROJECT 

New  York  city  obtains  its  chief  water  supply  from  the  Croton 
river  watershed.  Other  sources1  now  drawn  upon  are  less  important 
although  some  of  them,  such  as  the  Long  Island  underground 
supply,  are  capable  of  considerable  additional  development.  The 
average  daily  consumption  of  Croton  water  was  approximately 
324,ooo,ooo2  gallons  for  1907.  At  the  present  rate  of  increase  of 
population  the  consequent  daily  increase  in  consumption  of  water 
is  15,000,000  gallons  in  each  succeeding  year. 

The  entire  daily  flow  of  water  in  the  Croton  river  for  the  18 
years  from  1879  to  l%97  averaged  only  348,000,000  gallons.  About 
10,000,000  gallons  per  day  is  lost  by  evaporation  and  seepage 
from  existing  reservoirs.  The  records  for  40  years,  from  1868  to 
1907  make  a  somewhat  better  showing.  Making  no  allowance  for 
evaporation  the  average  flow  amounts  to  402,000,000  gallons.  With 
due  allowance  for  evaporation,3  however,  this  only  increases  the 
daily  supply  as  now  planned  by  about  47,000,000  gallons.  That  is, 
the  possible  total  additional  water  within  the  Croton  watershed 
would  suffice  for  only  three  years'  growth  of  the  city.  Much  of 
this  additional  water  belongs  to  periods  of  excessive  precipitation. 
To  save  it  would  require  additional  storage  facilities  for  3,05,000,- 
000,000  gallons,  and,  it  is  estimated,  would  probably  cost  $150,- 
000,000. 

1  Brooklyn  is  in  part  supplied  by  these  additional  sources  which  furnished 
145,000,000  gallons  daily  in  1007. 

2  The  figures  used  here  as  to  consumption  and  capacity  and  available 
supply  are  taken  from  the  printed  statements  of  the  commissioners  of  the 
New  York  City  Board  of  Water  Supply  in  a  circular  dated  April  16,  1908, 
and  are  based  upon  the  investigation  and  reports  of  the  corps  of  engineers 
headed  by  J.  Waldo  Smith,  chief  engineer,  John  R.  Freeman  and  William 
H.  Burr,  consulting  engineers.  The  reports  of  this  commission  and 
various  others  that  have  had  the  responsibility  of  investigating  the  future 
supplies  for  New  York  city  have  been  drawn  upon  freely  for  such  data. 

3  The  average  rainfall  for  the  past  40  years  is  about  49  inches  per  year. 
Only  about  48  per  cent  of  this  runs  into  the  streams.  The  rest  evap- 
orates or  is  absorbed  by  the  vegetation  or  joins  underground  supplies 
that  do  not  again  appear  at  the  surface  in  the  district. 

[9] 


[O 


NEW  YORK  STATE  MUSEUM 


Taking  into  account  the  small  relief  possible  in  this  direction  and 
the  certainty  that  in  less  than  five  years  the  demands  of  the  city 
will  be  greater  than  the  total  capacity  of  the  Croton  watershed,  it 
is  clear  that  some  other  source  of  large  and  permanent  supply  is  an 
absolute  necessity. 

In  the  search  for  such  additional  sources,  there  has  been  much 
careful  work  done  by  able  commissioners.1  In  the  meantime,  resi- 
dents of  certain  districts  where  there  are  possible  supplies  have 
taken  steps  by  legislative  action  to  effectually-  prevent  New  York 
city  encroaching  upon  their  territory.  Criticisms3  of  all  kinds 
largely  by  those  only  partially  informed  as  to  the  magnitude  and 
complexity  of  the  problem  and  partly  by  those  ignorant  of  the 
simplest  factors  in  its  solution,  have  been  kept  perpetually  before 
the  public.  One  needs  only  a  slight  acquaintance  with  such  public 
works  to  realize  that  it  is  much  easier  and  more  common  to  criticize 
and  raise  the  cry  of  corruption  or  incompetence  than  it  is  to  give 
really  valuable  advice  or  solve  a  real  problem  or  carry  an  enterprise 
i «f  the  most  vital  public  importance  to  a  successful  issue. 

It  is  sufficient  here  to  observe  that  exhaustive  studies  of  the  whole 
question  of  water  supply  by  competent  men  have  resulted  in  a 
practically  unanimous  conclusion  that  the  streams  of  the  Catskill 
mountains  are  the  most  satisfactory,  economical,  reliable,  abundant 
and  available  future  source  of  water. 

1  The  Report  of  John  R.  Freeman  C.  E.,  1899-1900;  Report  of  the  Burr- 
Herring-Freeman  Commission,  1902-4;  the  Studies  of  the  Department  of 
Water  Supply,  Gas  and  Electricity,  1902-4 ;  Investigations  of  the  Board  of 
Water  Supply,  1905  to  the  present  time. 

2  Acts  of  the  Legislature  of  1903-4. 

3  The  commonest  suggestions  neglect  the  question  of  permanence  or 
constancy  of  supply.  The  following  sources  are  often  mentioned,  (a)  Lake 
George,  forgetting  that  this  beautiful  lake  has  an  abnormally  small  water- 
shed and  could  never  figure  as  a  large  permanent  s upply ;  (b)  artesian 
wells,  ignoring  the  fact  that  with  the  exception  of  certain  portions  of  Long 
Island  there  is  almost  no  artesian  capacity,  and  on  Manhattan  and  the 
mainland  the  crystalline  rocks  make  such  development  useless;  (c)  Lake 
Ontario,  apparently  overlooking  the  great  distance  (400  miles)  and  the 
many  other  complications  that  this  international  water  body  involves; 

(d)  the  Housatonic  river,  neglecting  the  difficulties  of  interstate  origin ; 

(e)  Dutchess  county,  where  the  city  is  prohibited  by  legislative  enactment; 
(0  the  Hudson  river,  ignoring  the  fact  that  the  Hudson  is  an  estuary 
of  the  sea  with  brackish  water  of  a  very  impure  quality  and  wholly  unfit 
for  domestic  uses.  It  is,  however,  worth  while  to  note  that  Hudson  river 
water  is  sure  to  be  used  more  and  more  extensively  for  fire  protection  and 
similar  purposes  in  the  more  densely  populated  portions  of  the  city  by 
means  of  an  entirely  different  system  of  conduits.  This  is  one  of  the 
most  promising  directions  of  relief  looking  to  the  more  distant  future. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


I  I 


The  Catskill  supply  will  furnish  over  500,000,000  gallons  of 
.  water  daily  and  was  estimated  to  cost  $161,857,000.    That  is,  the 
additional  supplies  from  the  Catskills  as  planned  will,  when  com- 
pleted, be  sufficient  for  the  increasing  demands  of  the  growing  city, 
;  for  the  next  35  years.   And  some  of  it  may  be  badly  needed  long 
before  it  can  possibly  be  delivered. 

Parts  of  the  Catskill  system1 

The  chief  sources  within  the  Catskills  now  included  in  the  plans 
of  the  board  are : 

1  Esopus  creek,  to  be  taken  at  a  point  near  Olive  Bridge. 

2  Rondout  creek,  to  be  taken  at  a  point  near  Napanoch. 

3  Three  small  Streams  tributary  to  the  Rondout. 

4  Schoharie  creek,  to  be  taken  at  a  point  near  Prattsville. 

5  Catskill  creek,  to  be  taken  at  a  point  about  1  mile  northeast  of 
Durham. 

6  Six  small  streams  tributary  to  the  aqueduct  between  Catskill 
creek  and  Ashokan  reservoir. 

The  comparative  areas  of  watershed  and  their  daily  capacity  are 
estimated2  by  the  corps  of  engineers  as  follows : 


AREA  IN 
SQUARE 
MILES 


STORAGE  IN 
GALLONS 


DAILY  SUPPLY 
IN  GALLONS 


1  Esopus  watershed  

2  Rondout  watershed .  .  . 

3  Three  small  tributaries 

4  Schoharie  watershed . . . 

5  Catskill  watershed 

6  Six  small  streams  

Total  


255 
131 

45 
228 
163 

82 


70  000  000  000s 

20  000  000  000 

45  000  000  000 

30  000  000  000 


250  000  000 

98  000  000 

27  000  000 

136  000  000 

100  000  000 

49  000  000 


904 


165  000  000  000 


660  000  000 


1  The  subdivisions  and  proposed  locations  given  here  are  taken  chiefly 
from  the  Report  of  the  Board  of  Water  Supply  of  the  City  of  New  York 
to  the  Board  of  Estimate  and  Apportionment,  October  9,  1905. 

2  Estimates  are  much  more  complete  for  the  Esopus,  which  it  is  planned 
to  develop  first,  than  for  any  other  streams ;  and  it  must  be  understood 
that  the  figures  are  subject  to  revision  dependent  upon  modifications  of 
original  plans  to  meet  the  conditions  that  develop  upon  more  elaborate 
investigation. 

3  Preparations  are  to  be  made  for  storage  of  120,000,000.000  gallons  of 
water  on  the  Esopus,  but  a  part  of  this  capacity  is  intended  to  accommodate 
supplies  drawn  from  other  sources  than  Esopus  creek  itself. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


13 


The  evident  certainty  that  present  supplies  from  the  Croton  and 
Long  Island  will  be  very  inadequate  long  before  the  Catskill  system 
can  be  completed  has  influenced  the  adoption  of  plans  contemplating 
the  construction  of  certain  parts  in  advance  of  the  rest.  To  begin 
with,  only  the  Esopus  watershed  is  to  be  developed  by  the  con- 
struction of  the  great  Ashokan  dam  at  Olive  Bridge  making  the 
reservoir  of  full  capacity.  At  the  same  time  that  portion  of  the 
aqueduct  between  the  Ashokan  dam  and  the  present  Croton  reser- 
voir is  to  be  completed  in  advance  of  other  parts  so  as  to  make  it 
possible  to  turn  additional  supplies  into  the  Croton  system,  the 
capacity  of  the  present  Croton  aqueducts  being  somewhat  in  excess 
of  the  Croton  storage  in  dry  years.  It  is  furthermore  desirable  that 
increased  storage  capacity  should  be  secured  much  nearer  to  New 
York  city,  and  with  that  end  in  view  Kensico  reservoir  is  to  be 
greatly  enlarged.  It  is  estimated  that  this  may  be  made  to  hold  50 
days'  supply  of  500,000,000  gallons  daily. 

The  development  of  the  Catskill  system  is  being  carried  on  by  the 
Board  of  Water  Supply,  which  was  appointed  by  Mayor  McClellan, 
as  provided  in  chapter  724,  of  the  laws  of  1905.  The  present  board 
consists  of  John  A.  Bensel,  president,  Charles  N.  Chadwick  and 
Charles  A.  Shaw.  The  Engineering  Bureau  of  the  Board  is  in 
charge  of  J.  Waldo  Smith,  as  chief  engineer,  Merritt  H.  Smith,  as 
deputy  chief  engineer  and  Thaddeus  Merriman,  assistant  to  chief 
engineer. 

Influenced  doubtless  in  large  part  by  the  unity  of  certain  portions 
of  the  project,  either  because  their  essential  engineering  features 
are  distinct,  or  because  their  construction  is  more  urgent,  or  in  order 
to  facilitate  the  work  of  supervision  of  so  great  an  undertaking,  the 
following  departments  have  been  created : 

1  Headquarters  department  (executive).  In  charge  of  general 
designs,  plans  of  construction  and  preparation  of  contracts.  Alfred 
D.  Flinn,  department  engineer. 

2  Reservoir  department.  In  charge  of  development  of  the  Cats- 
kill  watershed  and  the  construction  of  the  various  dams  and  res- 
ervoirs.   Carlton  E.  Davis,  department  engineer. 

3  Northern  aqueduct  department.  In  charge  of  the  construction 
of  full  capacity  aqueduct  from  the  Ashokan  dam  (60  miles)  to  Hunt- 
ers brook  in  the  Croton  system.  Robert  Ridgway,  department  engi- 
neer. 

4  Southern  aqueduct  department.  In  charge  of  the  construction 
of  full  capacity  aqueduct  from  Hunters  brook  in  the  Croton  system 


14 


NEW  YORK  STATE  MUSEUM 


to  Hill  View  reservoir  on  the  northern  limits  of  New  York  city 
and  of  the  storage  reservoirs  and  filtration  work.  Merritt  H.  Smith, 
and  more  recently  F.  E.  Winsor,  department  engineer. 

5  Long  Island  department,  in  charge  of  the  development  of  the 
underground  water  supply  of  Long  Island.  A  plan  looking  toward 
this  end  has  been  prepared  and  approved  by  the  city  authorities  and 
is  now  being  reviewed  by  the  State  Water  Supply  Commission. 

6  City  aqueduct  division.  In  charge  of  the  delivery  of  water 
from  Hill  View  reservoir  throughout  Greater  New  York.  Origi- 
nally in  charge  of  W.  W.  Brush,  now  under  Walter  E.  Spear,  as 
department  engineer. 

Departments  are  further  divided  into  "  divisions  "  each  in  charge 
of  a  division  engineer  and  a  full  corps  of  assistants.  The  subdi- 
visions of  these  larger  units,  although  primarily  based  upon  con- 
venience and  efficiency  of  engineering  supervision,  coincides  rather 
closely  with  the  larger  geologic  problems  included  in  this  bulletin. 

I 

Generalities  of  construction 

The  chief  types  of  structure  projected  include  (i)  masonry  dams, 
(2)  earth  dikes  with  core  walls,  (3)  "cut  and  cover"  aqueduct 
through  country  of  about  the  elevation  of  hydraulic  grade,  (4) 
tunnels  through  mountains  or  ridges  that  are  too  high,  and  (5) 
pressure  tunnels  under  valleys  or  gorges  that  are  too  low. 

Some  of  these  are  of  record  proportions.  For  some  of  the  de- 
tails and  figures  see  the  different  special  problems  in  part  2. 

All  items  complete  as  planned  involve  a  total  of: 

10  dams 

10  impounding,  storage  and  distributing  reservoirs 
4.5  miles  of  dikes 

54.5  miles  of  "cut  and  cover"  aqueduct 
13.9  miles  of  tunnel  at  grade 
17.3  miles  of  pressure  tunnel  below  grade 
34     shafts  of  aggregate  depth  of  14,723  feet. 
6.3  miles  of  steel  pipes  making 

92.5  miles  of  aqueduct    complete    to    Hill    View  equalizing 
reservoir 
1      filtration  works 

18.0  miles  of  delivery  tunnel  in  New  York  city  to  the  terminal 

shafts  in  Brooklyn 
16.3  miles  of  delivery  pipe  lines 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


15 


Allowing  for  contingencies  and  costs  for  engineering  supervision 
the  system  is  estimated  to  cost  $176,000,000  and  many  years  will 
be  required  for  its  completion.  The  present  plans,  however,  con- 
template only  the  immediate  development  of  the  Esopus  watershed, 
the  storage  reservoirs  near  the  city  and  the  main  aqueduct  to  the 
various  points  of  delivery  within  the  city  limits.  It  is  expected 
that  part  of  this  additional  supply  of  water  will  be  available  by  the 
year  1913,  or  early  in  1914. 


CHAPTER  II 


PROBLEMS  ENCOUNTERED  IN  THE  PROJECT 

W'hen  the  Ashokan  reservoir  is  filled  the  surface  of  the  stored 
waters  will  stand  590  feet  above  the  sea.  Hill  View  reservoir  on 
the  northern  borders  of  New  York  city  will  have  an  elevation  of 
295  feet.  The  distance  between  these  two  points  is  nearly  75  miles 
in  direct  line.  The  contour  of  the  country  and  other  exigencies 
of  construction  will  increase  this  to  approximately  92  miles.  A 
main  distributary  conduit  in  New  York  city  will  add  18  miles  more. 

The  destination  of  the  water  therefore  before  distribution  begins 
is  300  feet  lower  than  its  starting  point.  This  is  sufficient  head  to 
permit  gravitational  flow  and  a  self-delivering  system.  If  the  hy- 
draulic gradient  can  be  maintained  it  would  evidently  constitute  a 
decided  advantage.  The  plans  have  therefore  from  the  beginning 
contemplated  such  construction.  It  means  then  that  a  flowing 
grade  must  be  maintained  in  all  tunnels  or  channels  or  tubes  and 
that  when  a  depression  has  to  be  crossed  the  pressure  must  be 
maintained  in  some  sort  of  a  conduit  so  that  the  water  may  rise 
again  to  a  suitable  level  on  the  other  side. 

The  difficulties  of  accomplishing  this  in  a  work  of  such  magnitude 
are  not  at  first  apparent.  The  full  significance  of  the  undertaking 
can  be  realized  only  after  a  study  of  the  country  through  which  the 
aqueduct  must  be  carried.  It  then  resolves  itself  into  a  series  of 
problems,  each  one  having  its  own  characteristics  and  peculiar 
difficulties  and  methods  of  solution  and  each  requiring  a  thorough 
understanding  of  the  topographic  features  of  the  vicinity  and  a 
working  knowledge  of  geologic  conditions. 

General  questions 

It  is  sufficient  at  this  point  to  call  attention  to  the  facts  of  the 
topographic  map  and  point  out  only  the  most  general  physiographic 
features  that  may  at  once  be  seen  to  materially  modify  the  simplicity 
of  the  line. 

For  example,  one  has  scarcely  left  the  great  reservoir,  with  water 
flowing  at  580-90  feet  above  tide,  before  the  broad  Rondout 
valley  is  reached,  with  a  width  of  4*/.  miles  nowhere  at  great 
enough  elevation  to  carry  the  aqueduct  at  grade.  If  it  is  to  be 
crossed  at  all,  and  it  must  be  crossed  to  reach  New  York  city,  some 

[17] 


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NEW   YORK  STATE  MUSEUM 


special  means  must  be  devised.  If  a  trestle  be  proposed,  one  finds 
that  it  would  have  to  be  4>j  miles  long  (24,000  feet),  and  in  some 
places  300  feet  high,  and  at  all  points  large  enough  and  strong 
enough  to  carry  a  stream  of  water  capable  of  delivering  500,000,000 
gallons  daily  —  a  stream  that  if  confined  in  a  tube  of  cylindrical 
form  would  have  a  diameter  of  about  15  feet. 

A  steel  tube  might  be  laid  to  carry  the  water  across  and  deliver 
it  again  at  flowing  grade,  but  here  one  is  met  with  the  fact  that  it 
would  require  a  tube  of  unprecedented  size  and  strength  and  if 
divided  into  a  number  of  smaller  ones  the  cost  would  be  greater  than 
that  of  a  tunnel  in  solid  rock. 

The  other  alternative  is  to  make  a  tunnel  deep  enough  in  bed 
rock  to  lie  beneath  surface  weaknesses  and  superficial  gorges  and 
in  it  carry  the  water  under  pressure  to  the  opposite  side  of  the 
valley.  This  is  the  plan  that  seems  best  suited  to  the  magnitude 
of  the  undertaking  and  would  seem  to  promise  most  permanent  con- 
struction. But  no  sooner  is  this  conclusion  reached  than  it  is 
realized  that  there  are  now  several  hitherto  unregarded  features 
that  assume  immediate  and  controlling  importance.  Some  of  these, 
for  example,  are  (1)  the  possibility  of  old  stream  gorges  that  are 
buried  beneath  the  soil,  (2)  the  position  of  these  old  channels  and 
their  depth,  (3)  the  kinds  of  rock  in  the  valley,  (4)  their  character 
for  construction  and  permanence,  (5)  the  possible  interference  of 
underground  water  circulation,  (6)  the  possible  excessive  losses  of 
water  through  porosity  of  strata,  (7)  the  proper  depth  at  which  the 
tunnel  should  be  placed,  (8)  the  kinds  of  strata,  and  their  respective 
amounts  that  will  be  cut  at  the  chosen  depth,  (9)  the  position  and 
character  of  the  weak  spots  with  an  estimate  of  their  influence  on 
the  practicability  of  the  tunnel  proposition.  Then  after  these  have 
all  been  considered  the  whole  situation  must  be  interpreted  and 
translated  into  such  practical  engineering  terms  as  whether  or  not 
the  tunnel  method  is  practicable,  and  at  what  point  and  at  what 
depth  it  should  cross  the  valley,  and  at  what  points  still  further 
exploration  would  add  data  of  value  in  correcting  estimates  and 
governing  construction  and  controlling  contracts. 

This  is  a  general  view  of  one  case,  the  first  one  of  any  large 
proportions  in  following  down  the  aqueduct.  •  There  are  many 
others.  In  nearly  all  of  them  the  importance  of  geologic  questions 
is  prominent.  Many  of  them,  of  course,  are  of  the  simplest  sort, 
but,  on  the  other  hand,  some  are  among  the  most  obscure  and 
evasive  problems  of  the  science.    And  they  do  not  become  any 


/ 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


19 


easier  simply  to  know  that  they  must  ultimately  be  stated  in  terms 
precise  enough  for  the  use  of  engineers,  and  to  know  furthermore 
that  the  real  facts  are  to  be  laid  bare  when  construction  begins  and 
as  it  progresses.  But  from  another  viewpoint  it  may  be  regarded 
as  an  exceptionally  fine  opportunity  to  study  applied  geology  in  its 
best  form  and  to  see  the  intimate  interrelationship  between  an 
engineering  enterprise  of  great  public  utility  and  a  commonly  con- 
sidered more  or  less  obscure  science.  The  services  of  geology  have 
been  seldom  so  consistently  employed  in  earlier  undertakings  of 
similar  character.  It  is  to  be  hoped  that  the  accompanying  illus- 
trations of  the  practical  application  of  geologic  knowledge  and  facts 
to  engineering  plans  and  practice  may  add  to  the  appreciation  of 
the  commonness  and  variety  of  such  service  in  many  everyday 
affairs.  Furthermore,  this  unique  enterprise,  the  like  of  which  for 
magnitude  and  complexity  has  never  before  been  attempted,  has 
given  to  those  whose  good  fortune  has  brought  them  into  working 
relations  with  its  problems,  the  opportunity  of  a  generation  in  their 
chosen  field.1  The  success  stages  from  isolated  observations, 
inference,  hypothesis,  theory,  conclusions,  and  fully  proven  facts 
are  all  represented.  The  steps  more  or  less  fully  coincide  with  the 
degree  of  confidence  observable  in  the  tone  of  advisory  reports  to 
the  engineers  in  charge  —  representing  suggestions,  recommenda- 
tions, or  specific  advice. 

Tt  is  one  of  the  cherished  wishes  of  the  writer  of  this  bulletin 
that  some  of  these  problems  may  be  presented  in  such  manner  as 
to  serve  a  distinct  educational  purpose.  For  this  reason  in  part, 
deeming  it  even  of  greater  importance  than  the  mere  enumeration 
of  newly  discovered  facts,  the  writer  has  chosen  to  treat  the  sub- 
ject from  the  standpoint  of  an  instructor  illustrating  the  develop- 
ment of  working  conclusions.  Tt  is  certain  that  not  all  readers  have 
the  same  degree  of  preparation  or  acquaintance  with  the  subject- 
matter,  and  it  may  therefore  be  useful  to  include  many  things  that 
some  may  well  pass  by.  No  excuse  is  offered  except  that  such 
method  of  treatment,  in  behalf  of  the  general  intelligent  public  that 
it  is  hoped  to  reach,  seems  to  the  author  to  be  advisable. 

1  W.  O.  Crosby  of  the  Massachusetts  Institute  of  Technology,  James 
F.  Kemp  and  Charles  P.  Berkey  of  Columbia  University  have  constituted 
tlie  staff  of  consulting-  geologists  throughout  most  of  the  exploratory  work. 


20 


NEW  YORK  STATE  MUSEUM 


Other  problems 

The  foregoing  observations  apply  likewise  to  the  other  larger 
problems  of  the  aqueduct  line.  A  list  of  the  larger  ones  requiring 
extensive  exploration  and  illustrating  geologic  application  in  their 
solution  are  given  below : 

1  Location  of  the  Ashokan  dam 

2  Sources  of  material  for  construction 

3  Crossing  the  Rondout  valley 

4  The  Wallkill  valley 

5  Moodna  buried  valley 

6  Pagenstechers  gorge  and  Storm  King  mountain 

7  The  Hudson  river  crossing  problem 

8  The  Storm  King-Break  Neck  cross  section 

9  Foundry  brook 

10  Sprout  brook  notch 

1 1  Peekskill  creek  valley 

12  Croton  lake  pressure  tunnel 

13  Bryn  Mawr  siphon 

14  The  new  Kensico  dam 

15  Kensico  quarries 

16  New  York  city  delivery  tunnel 

In  addition  to  these  there  are  several  questions  of  general  bear- 
ing in  which  the  chief  lines  of  argument  and  the  chief  basis  of  con- 
clusion are  essentially  geologic.  Although  little  wholly  new  data  is 
yet  available  on  these  particular  questions  from  any  direct  work  of 
the  aqueduct,  yet  it  will  add  materially  to  an  appreciation  of  the 
far-reaching  influence  of  established  geologic  data  and  geologic  rea- 
soning to  enumerate  some  of  them : 

17  Continental  subsidence  and  elevation 

18  Crustal  warping 

19  Postglacial  and  present  faulting 

20  Underground  water  circulation 

21  Relative  resistance  of  the  different  formations  to  corrosion  by 
aqueduct  waters 

22  Structural  materials 

Each  of  these  problems  or  questions  or  topics  is  discussed  sepa- 
rately, so  far  as  practicable.  By  adopting  this  plan,  of  course  there 
is  a  tendency  to  repetition  but  this  to  a  certain  extent  is  unavoid- 
able.   Some  of  it  is  overcome  by  suitable  references  to  preceding 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


21 


discussions.  Where  such  cross  reference  is  too  cumbersome,  the 
items  are  repeated  in  preference  to  leaving  the  case  obscure.  Thus 
it  is  hoped  to  make  each  case  a  unit,  and  the  whole  series  useful 
and  understandable. 

Gathering  data 

In  the  accumulation  of  data  all  the  members  of  the  engineering 
corps1  as  well  as  the  men  acting  only  in  a  consulting  capacity  have 
taken  part.  Necessarily  the  bulk  of  the  exact  data  has  been 
gathered  by  the  men  all  the  time  on  the  ground  and  whose  duty  it 
was  to  superintend  explorations.  The  care  and  intelligence  with 
which  this  has  been  done  is  notable.  A  considerable  proportion  of 
the  labor  of  manipulating  the  accumulated  data  and  interpreting  it 
so  as  to  reach  an  explanation  of  conditions  and  formulate  conclu- 
sions has  been  assumed  by  the  consulting  men. 

Too  much  credit  can  not  be  given  to  the  heads  of  departments 
and  divisions  for  the  open-handed  way  in  which  all  needed  facts 
were  held  available  at  all  times  for  comparison  and  guidance  toward 
sound  conclusions.  The  information  upon  which  investigations 
have  been  initiated  have  been  chiefly  the  following: 

1  The  geologic  maps  and  reports  of  the  New  York  State  Survey 

2  United  States  topographic  maps 

3,  Geologic  folio  no.  83,  New  York  city  folio 

4  Earlier  engineering  records  and  reports 

5  Reports  of  special  commissions  on  water  supply 

1  In  this  work,  no  group  of  men  have  had  so  direct  responsibility  as  the 
division  engineers.  The  success  with  which  so  many  complicated  explora- 
tions were  carried  out  is  chiefly  due  to  their  constant  care  and  foresight 
and  perseverance  and  the  able  assistance  of  their  staff.  Those  who  have  had 
especially  important  divisions  for  the  geological  problems  involved  are  given 
due  credit  in  the  discussions  of  part  2,  of  this  bulletin.  It  is  easy,  how- 
ever, to  neglect  sufficiently  full  acknowledgment  of  their  services  in  gather- 
ing and  formulating  data  of  this  kind.  Among  those  having  charge  of  the 
most  important  exploratory  work  the  following  names  should  appear: 

James  F.  Sanborn,  for  sometime  assigned  to  geologic  work  on  the  North- 
ern aqueduct. 

William  E.  Swift,  in  charge  of  the  Hudson  river  explorations. 
William  W.  Brush,  in  charge  of  the  early  New  York  city  explorations. 
Lazarus  White,  in  charge  of  the  Rondout  valley  explorations. 
Lawrence  C.  Brink,  in  charge  of  the  Wallkill  division  explorations. 
J.  S.  Langthorn,  in  charge  of  the  exploratory  work  at  the  Ashokan  Reser- 
voir. 

Wilson  Fitch  Smith,  in  charge  of  work  at  Kensico  dam,  and 
A.  A.  Sproul,  in  charge  of  the  Peekskill  creek  and  Sprout  brook  explora- 
tions. 


22 


NEW    YORK  STATE  MUSEUM 


Some  of  these  are  printed  reports  and  records  not  directly  con- 
cerned with  this  enterprise,  but  whose'  information  has  been  found 
useful  in  this  field.  This  is  especially  true  of  the  first  four  sources 
enumerated,  i,  2,  3,  4.  The  last  is  a  specific  study  with  direct 
reference  to  this  project. 

Investigations  were  begun  from  the  above  vantage  point.  The 
methods  employed  and  the  explorations  conducted  constituting  the 
further  sources  of  information  and  furnishing  the  complete  data 
upon  which  all  conclusions  have  been  based  include  the  following: 

6  Detailed  topographic  studies  of  the  engineers  of  the  Board  of 

Water  Supply 

7  Geologic  field  work  making  observations  in  detail  of  all  geo- 

logic factors  that  seem  to  bear  on  the  problem  in  hand 

8  Wash  borings  for  depth  to  bed  rock 

9  Chop  drill  holes  through  stony  ground  to  bed  rock 

10  Shot  drill  holes  in  bed  rock 

11  Diamond  drill  holes 

12  Test  pits  and  trenches  for  detail  of  drift  structure 

13  Test  tunnels  in  rock  for  working  quality 

14  Deflection  tests  for  holes  that  have  swerved  aside 

15  Pumping  tests  for  underground  water  supply 

16  Pressure  tests  for  rock  porosity 

17  Microscopic  examinations  of  rock  types 

18  Laboratory  tests  of  quality  and  behavior  of  materials. 

The  mass  of  data  accumulated  from  all  these  sources  is  surpris- 
ing. For  example,  there  are  upward  of  200  wash  borings  on  the 
different  proposed  Hudson  river  crossing  lines  alone ;  there  are  69 
drill  borings  and  177  wash  borings  on  the  site  of  Kensico  dam; 
there  are  69  shot  and  diamond  drill  holes  on  the  Rondout  siphon 
line  aggregating  10.234  feet  of  rock  core ;  there  are  65  drill  holes  of 
various  sorts  on  the  Moodna  creek  siphon  aggregating  in  total  pene- 
tration of  drift  over  10,000  feet:  there  are  106  borings,  besides 
several  pits  and  trenches  at  Ashokan  dam  location.  At  every  point 
explorations  suitable  to  the  particular  problems  in  hand  were  con- 
ducted. The  whole  mass  of  data  is  conveniently  recorded,  much  of 
it  is  tabulated,  some  of  it  is  represented  graphically,  samples  of 
nearly  all  of  the  material  are  available  for  examination.1  and  all 

1  The  cores  of  all  drillings  and  suitable  samples  of  all  borings  in  drift 
have  been  saved  and  properly  labeled  and  are  to  be  permanently  boused  at 
some  convenient  point  on  tbe  aqueduct  line  when  completed.  At  present 
they  are  cared  for  at  the  different  division  offices. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


23 


have  been  made  use  of  in  coining  to  a  consistent  understanding  of 
the  conditions. 

But  the  amount  of  accumulated  data  is  no  more  remarkable  than 
the  difficulties  that  have  been  encountered  in  obtaining  it.  For 
example,  in  the  Moodna  valley  it  has  taken  three  to  four  months' 
time  to  put  down  a  single  hole  to  bed  rock  —  the  average  time  con- 
sumed for  each  of  the  15  holes  exploring  the  deepest  portion  of 
the  valley  was  about  60  days.  The  chief  trouble  is  caused  by  heavy 
bouldery  till.  In  one  case  a  boulder  was  penetrated  for  35  feet, 
lying  a  hundred  feet  above  bed  rock. 

The  extreme  of  such  difficulty  is,  of  course,  encountered  in  the 
Hudson  river  itself,  where  the  drill  has  to  contend  with:  (1)  the 
rise  and  fall  of  the  tides,  (2)  the  river  currents,  (3)  a  maximum  of 
90  feet  of  water,  approximately  700  feet  of  silt,  gravel,  till,  boulders, 
etc.,  filling  the  old  preglacial  gorge.  The  heavy  steamboat  and 
towing  traffic  has  been  a  serious  element  in  the  problem.  Probablv 
never  anywhere  have  drillmen  had  to  face  so  nearly  insurmount- 
able obstacles.  In  two  years  only  two  holes  reached  below  a  depth 
of  600  feet  below  sea  level.  A  third,  now  in  progress,  has  pene- 
trated a  depth  of  768  feet  without  entering  rock. 


CHAPTER  III 

RELATIVE  VALUES  OF  DIFFERENT  SOURCES  OF  INFORMA- 
TION AND  STAGES  OF  DEVELOPMENT 

In  the  earlier  stages  of  work  topographic  features  were  of  most 
concern,  and  they  largely  controlled  the  selection  of  reservoir  sites 
and  possible  lines  for  the  aqueduct  to  follow.  It  was,  however, 
at  once  recognized  that  tunnels  would  be  unavoidable  and  studies 
as  to  the  types  of  rock  formations  to  be  encountered  were  begun 
It  was  also  early  appreciated  that  the  soil  or  drift  cover  is  very 
unevenly  distributed  over  the  rock  surface  and  that,  especially  in 
the  chief  valleys  requiring  pressure  tunnels,  it  would  be  necessary 
to  determine  the  profile  of  the  rock  floor.  At  this  point  wash  bor- 
ings were  begun.  But  the  natural  limitations  of  the  wash  rig1  for 
penetrating  drift  of  all  kinds  left  the  information  still  too  indefinite. 
The  wash  rig  can  not  penetrate  hard  rock.  It  can  not  wash  up 
anything  but  the  finer  matter,  and  a  boulder  of  very  moderate  size 
is  almost  as  effectual  a  barrier  as  true  rock  ledge.  By  a  combination 
of  washing  and  chopping  or  by  the  use  of  an  explosive  to  break  or 
dislodge  an  obstruction  some  progress  in  unfavorable  material  may 
be  made,  but  the  wash  rig  alone,  in  a  drift-covered  region,  gives 
only  negative  results.  It  is  certain,  for  example,  that  bed  rock  lies 
at  least  as  deep  as  the  wash  rig  has  penetrated,  but  it  is  not  certain 
that  it  is  bed  rock  instead  of  some  other  obstruction.  Except  in 
areas  of  special  drift  conditions,2  therefore,  the  wash  rig  was  insuf- 
ficient. To  rely  upon  the  process  at  random  was  clearly  impossible, 
and  to  determine  whether  or  not  the  results  of  a  particular  locality 

1  A  "  wash  rig "  is  a  device  composed  essentially  of  two  iron  pipes,  one 
within  the  other,  and  so  mounted  that  the  inner  one  can  be  worked  up 
and  down  in  sort  of  a  churning  fashion  while  water  under  considerable 
pressure  is  forced  through  it  to  the  bottom  and  out  again  by  the  larger 
pipe  to  the  surface,  carrying  up  with  the  current  the  displaced  sand  and 
clay.  As  progress  is  made  with  the  inner  pipe  the  outer  one  is  from  time 
to  time  driven  down  and  the  process  renewed  and  repeated  till  the  hole  is 
finished. 

2  One  of  the  most  notable  areas  of  special  drift  conditions  is  repre- 
sented in  the  Walkhill  valley  Isee  discussion  in  pt  2]  where  there  were 
developed  large  deposits  of  modified  drift,  stratified  gravel,  sand  and  clays, 
lying  immediately  upon  the  bed  rock  floor.  In  this  the  wash  bore 
process  was  eminently  satisfactory,  and  the  rapid  progress  made  by  it 
together  with  its  economy  made  this  an  especially  attractive  method  of 
exploration. 

[25] 


26 


NEW  YORK  STATE  MUSEUM 


cuiild  be  relied  upon  became  involved  at  once  with  an  interpretation 
of  local  glacial  phenomena,  especially  an  interpretation  of  the  char- 
acter of  the  local  drift.  In  order  to  see  the  limited  application  of 
this  method  one  needs  only  to  point  out  that  the  majority  of  drift 
deposits  in  this  region  are  stony  or  even  bouldery,  forming  thick 
coverings  in  the  valleys,  and  to  call  attention  to  the  experience  at 
two  or  three  points.  For  example,  at  Moodna  creek,  the  prelimi- 
nary wash  borings  were  obstructed  and  bed  rock  reported  at  5  to 
15  feet  below  the  surface  where  afterward,  by  other  means,  it  was 
proven  to  lie  more  than  300  feet  down.  Or  again,  in  the  pre- 
liminary wash  borings  in  the  Hudson,  the  rigs  were  stopped  and 
rock  bottom  provisionally  reported  at  from  25  to  200  feet  below 
sea  level,  but  later  explorations  have  proven  at  the  same  point  that 
rock  bottom  is  more  than  700  feet  down. 

Therefore,  to  the  "  wash  rig  "  was  added  the  "  chop  drill  "  and 
the  "  oil-well  rig  "  and  to  these,  or  to  modifications  of  them,1  the 
success  in  reaching  bed  rock  has  been  due. 

From  independent  field  studies  of  a  more  strictly  geologic  nature 
it  became  clear  that  many  of  the  valleys,  where  pressure  tunnels 
were  proposed,  are  of  comparatively  complex  geologic  structure  and 
exhibit  considerable  variety  of  rock  quality  and  condition.  This 
then  introduced  and  necessitated  still  more  elaborate  lines  of  ex- 
ploration. It  was  not  enough  to  know  the  profile  of  rock  floor 
alone,  it  became  of  equal  importance  to  penetrate  the  rock  and  obtain 
samples  of  it.  So  the  shot  drill 2  and  the  diamond  drill 3  were 
employed  and  the  drill  cores  preserved  for  identification  and 
reference. 

1  The  essential  features  of  the  machines  in  most  instances  are,  a  high 
tower  or  support,  a  heavy  chisel-shaped  plunger  that  can  be  raised  by 
a  rope  and  dropped  repeatedly  in  the  hole,  destroying  or  displacing 
obstructions,  and  which  can  be  followed  by  a  casing  driven  down  as 
progress  is  made  —  a  combination  of  washing,  chopping  and  driving. 

2  The  shot  drill,  or  calyx  drill,  is  essentially  a  machine  devised  to  rotate 
a  steel  tube  which  is  so  adjusted  and  manipulated  that  a  supply  of  small 
chilled  shot  can  be  kept  continually  under  the  lower  end  as  it  bores  into 
the  rock.  The  cutting  is  done  by  the  shot  immediately  under  the  edge 
of  the  tube.  A  core  remains  in  the~  tube  and  may  be  recovered.  Its  best 
position  is  vertical. 

3  The  diamond  drill  consists  essentially  of  a  bit  or  crown  set  with  black 
diamonds  (bort)  in  such  manner  that  when  the  bit  is  attached  to  a  rotating 
tube  a  circular  groove  is  cut  into  the  rock.  By  proper  attachment  to 
jointed  tubes  and  driving  gear  a  hole  may  thus  be  bored  at  any  angle  and 
to  great  depth  and  a  core  recovered. 


GEOLOGY  OF  THE  NEW   YORK  CITY  AQUEDUCT 


27 


These  preserved  cores,  now  aggregating  many  thousands  of  feet 
have  been  of  great  service  in  determining  the  precise  limits  of 
formations  and  consequently  the  geologic  structure  or  cross  section, 
by  which  detailed  estimates  may  be  guided. 

Even  these  occasionally  appeared  to  give  insufficient  data.  The 
peculiar  behavior  of  certain  holes,  as,  for  example,  one  or  more 
at  Foundry  brook,1  led  to  the  suspicion  that  the  drill  had  swerved 
from  its  course,  following  a  particularly  soft  seam  or  zone,  and 
that  the  results  secured  by  it  without  large  corrections,  were  wholly 
misleading.   Tests  proved  that  there  had  been  a  deflection. 

At  this  and  many  other  places  it  later  became  very  desirable  to 
form  some  quantitative  as  well  as  qualitative  opinion  of  the  condi- 
tions existing  in  the  underlying  strata.  The  percentage  of  core 
saved,  the  rate  of  progress  of  the  drill,  the  behavior  of  the  drill,  the 
condition  of  the  core  recovered,  the  loss  of  water  in  the  hole  —  all 
these  of  course  were  considered. 

For  more  definite  evidence  as  to  porosity  and  perviousness,  a 
series  of  carefully  planned  pressure  tests2  were  made.  By  shutting 
off  connection  with  the  walls  of  the  hole  above  a  certain  stratum 
and  forcing  water  in  under  pressure,  it  was  possible  to  demonstrate 
that  certain  strata  or  certain  portions  were  practically  impervious 
in  their  natural  bed,  while  others  were  much  less  so,  and  to  get  an 
idea  of  their  relative  efficiency  as  water  carriers.  For  the  pressure 
tunnels,  especially,  this  test  is  a  very  suggestive  line  of  investigation. 

1  At  Foundry  brook  \sce  discussion  of  this  problem  in  pt  2],  the  remark- 
able condition  apparently  shown  was  a  reasonably  substantial  ledge  of 
granitic  gneiss,  50  feet,  followed  below  by  200  feet  of  apparently  soft 
sand  and  reported  as  such.  No  core  could  be  recovered.  So  extensive 
a  zone  or  bed  or  layer  or  mass  is  hardly  conceivable  considering  the 
crystalline  silicious  character  of  the  rock.  Tt  probably  represents  a  steeply 
dipping  crush  zone  along  fault  movement  where  the  increased  underground 
circulation  has  been  unusually  effective  in  producing  decay.  After  enter- 
ins  this  zone  the  drill  swerved  from  its  initial  course  and  kept  within  the 
soft  seam. 

2  The  pressure  test  is  made  by  means  of  a  force  pump,  fitted  with  a 
gage  on  which  the  pressure  is  recorded,  connected  by  a  pipe  to  the  por- 
tion of  the  hole  to  be  tested,  and  so  adjusted  to  a  device  for  blockading; 
or  damming  the  hole  that  the  water  pressure  is  confined  to  those  portions 
of  the  walls  of  the  hole  below  the  dam,  or  between  two  dams  if  an  upper 
and  lower  one  are  used.  In  this  way  any  portion  of  a  hole,  or  stratum  or 
several  beds  together  may  be  tested  and  the  amount  of  water  absorbed 
per  unit  of  time  per  unit  of  pressure  determined.  This  is,  of  course, 
directly  related  to  the  porosity  of  the  rock  and  is  approximately  inversely 
proportional  to  its  presumed  value  as  an  aqueduct  carrier. 


28 


NEW  YORK  STATE  MUSEUM 


Where  the  strata  are  especially  porous  and  where  underground  or 
permanent  ground  water  supplies  are  very  extensive  and  where  at 
the  same  time  the  largest  or  deepest  pressure  tunnels  are  projected 
some  uneasiness  has  been  entertained  as  to  the  extent  of  interference 
from  inflowing  water  during  construction.  An  attempt  to  form 
some  idea  of  the  ease  of  such  underground  circulation  has  been 
made  by  a  systematic  pumping  of  one  or  two  critical  holes.  The 
results  leave  many  factors  still  too  obscure  to  draw  definite  con- 
clusions. The  test  will  be  taken  up  again  in  the  discussion  of  the 
Rondout  siphon  in  part  2. 

Laboratory  tests  and  experiments  on  materials  complete  the  list 
of  lines  of  investigation  with  which  this  bulletin  is  concerned. 
Although  from  the  nature  of  the  case  these  are  elaborate  and 
unusually  complete,  the  more  important  lines  are  not  at  all  new. 
All  the  methods  of  petrographic,  chemical,  and  physical  manipula- 
tion that  seem  to  promise  practical  results  of  value  to  the  success 
of  the  undertaking  are  followed  and  the  data  are  organized  and 
interpreted  and  conclusions  are  formulated  with  as  great  definite- 
ness  for  practical  bearing  as  other  lines  of  investigation. 


CHAPTER  IV 


GENERAL  GEOLOGY  OF  THE  REGION 

It  will  save  much  repetition  and  it  is  believed  will  altogether 
serve  a  useful  purpose  in  maintaining  unity  of  treatment  to  give 
an  outline  of  the  geologic  features  of  the  region  in  advance  of  the 
discussion  of  special  problems.  It  is  intended  only  for  those  not 
sufficiently  familiar  with  the  general  geology  to  follow  subsequent 
matters. 

The  region  includes  some  of  the  most  complicated  and  obscure 
sections  of  New  York  geology.  It  is  simple  in  almost  no  one  of  the 
larger  branches  of  the  subject.  In  physiography  there  is  the  long 
and  involved  history  and  the  results  of  long  continued  erosion  of  a 
variable  series  of  formations  in  different  stages  of  modification  as 
to  structure  and  metamorphism  and  attitude,  modified  still  fur- 
ther by  subsidences  and  elevations,  depositions  and  denudations, 
peneplanations  and  rejuvenations,  glaciation  and  recent  erosion  — 
all  together  introducing  as  much  complexity  as  can  well  be  found  in 
a  single  area. 

In  stratigraphy  the  whole  range  of  the  eastern  New  York  geologic 
column  is  represented  from  the  oldest  known  formation  up  to  and 
including  the  Middle  Devonic  —  a  succession  of  at  least  25  distinct 
formations  which  may  for  convenience  be  treated  in  groups  that 
have  had  similar  history.  Each  of  these  formations  has  a  constant 
enough  character  to  map  and  regard  as  a  physical  unit.  Even  this 
classification  ignores  the  great  range  of  petrographic  variability 
shown  in  such  formations  as  the  Highlands  or  Fordham  gneisses. 
All  but  two  or  three  of  these  formations  will  be  cut  by  the  tunnels 
of  the  aqueduct. 

In  petrography  the  range  is  even  greater  —  so  great,  in  fact,  that 
only  an  enumeration  of  the  variations  will  be  attempted.  They 
include  elastics,  metamorphics  and  igneous  types ;  stratified  and  un- 
assorted, coarse  and  fine,  detrital  and  organic,  marine  and  fresh 
water,  homogeneous  and  heterogeneous,  argillaceous,  calcareous  and 
silicious  sediments,  unmodified  and  thoroughly  recrystallized  strata ; 
acid  and  basic  and  intermediate  intrusions ;  massive  and  foliated 
crystallines  —  of  many  varieties  or  variations  in  each  group. 

In  tectonic  geology  an  equal  complexity  prevails.  There  are  regu- 
lar stratifications,  cross-beddings,  disconformities,  overlaps  and  un- 
conformities ;  interbeddings,  lenses  and  wedges ;  flat,  warped,  tilted 

[29] 


NEW   YORK  STATE  MUSEUM 


and  crumpled  strata;  monoclinal  and  isoclinal,  open  and  closed, 
anticlinal  and  synclinal,  symmetrical  and  overturned,  horizontal  and 
pitching  folds ;  joints,  crevices,  caves,  crush  zones,  shear  zones,  and 
contacts;  normal,  thrust,  dip,  strike,  large  and  small  faults;  veins, 
segregations,  inclusions,  dikes,  sills,  bosses  and  bysmaliths. 

With  such  variety  of  natural  conditions  it  is  not  surprising  that 
the  problems  of  the  aqueduct  are  also  of  great  variety.  No  two 
have  in  all  respects  the  same  factors  in  control  and  no  two  can  be 
explored  and  interpreted  upon  exactly  the  same  lines. 

i  Geographic  features  or  districts.   (Physical  geography1) 

It  will  be  convenient  at  this  point  to  think  of  the  surface  topog- 
raphy by  districts  —  not  wholly  distinct  from  each  other,  but  still 
with  essential  differences  of  origin  and  form.  From  south  to  north 
they  are:  (a)  New  York-Westchester  county  district.  The  area 
of  crystalline  sediments.  South  of  the  Highlands,  (b)  Highlands 
of  the  Hudson  (Putnam  county),  (c)  Wallkill-Newburgh  district. 
From  the  Highlands  to  the  Shawangunk  range,  (d)  Shawangunk 
range  and  Rondout  valley,    (e)    Southern  Catskills. 

All  have  been  sculptured  by  the  same  forces  and  with  similar 
vicissitudes,  but  the  difference  of  history  and  structure  and  condi- 
tion, already  established  when  the  physiographic  forces  began  on  the 
work  now  seen,  have  caused  the  variety  of  surface  features  indi- 
cated in  the  divisions  made  above.  The  more  noticeable  character- 
istics of  these  five  districts  are  here  given. 

a  New  York- Westchester  district.  The  area  south  of  the 
Highlands  proper  is  characterized  by  a  comparatively  regular  suc- 
cession of  nearly  parallel  ridges  separated  by  valleys  of  nearly  equal 
extent  (y2  to  5  miles  wide),  making  a  surface  of  gently  fluted 
aspect  and  of  moderate  relief  (0-500  feet)  sloping  endwise  toward 
the  Hudson  and  the  sea.  The  controlling  factors  in  producing  this 
topography  are  involved  in  a  series  of  folded,  foliated,  crystalline 
sediments,  of  differing  resistance  to  destructive  agencies. 

b  The  Highland  region  is  one  of  rugged  features,  with  a 
range  of  elevation  of  0-1600  feet  A.  T.,  forming  mountain  masses 
and  ridges  separated  by  very  narrow  valleys  all  having  a  general 
northeast  and  southwest  trend  across  which  the  Hudson  cuts  its 
way  in  a  narrow,  angular  gorge,  forming  the  most  constricted  and 
crooked  portion  of  its  lower  course.   The  bed  rock  is  all  crystalline, 

1  The  physiographic  history  of  a  region  is  not  understandable  without  a 
comprehensive  knowledge  of  its  geologic  features  and  structures  and  history. 
Tt  is  therefore  treated  in  a  later  paragraph. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


3> 


of  massive  and  foliated  types,,  metamorphosed  sediments  in  part 
with  large  masses  of  igneous  intrusions  and  bosses. 

c  The  Wallkill-Newburgh  district  lying  immediately  north 
of  the  Highlands  and  extending  to  the  Shawangunk  range  is  a 
region  of  gently  rolling  contour.  Most  of  the  area  along  the  pro- 
posed lines  lies  between  200  and  500  feet  above  the  sea.  There  are 
only  occasional  rugged  hills  or  short  ridges,  such  as  Snake  hill  and 
Skunnemunk.  The  valleys  are  broad  and  smooth  and  the  divides 
are  simply  broad,  hilly  uplands.  Bed  rock  is  chiefly  Hudson  River 
slates  with  occasional  belts  of  Wappinger  limestone.  The  larger 
features,  the  trend  of  divides  and  valleys,  are  northeast  and  south- 
west, although  this  regularity  is  not  so  marked  as  in  the  preceding 
two  districts.  But  the  chief  streams  flow  either  northeast  or  south- 
west to  the  Hudson  along  these  general  lines. 

d  The  Shawangunk  range  and  Rondout  valley  form  a 
transitional  unit  from  the  complicated  structural  and  tectonic  con- 
ditions of  the  southerly  districts  to  the  uniform  and  almost  undis- 
turbed strata  of  the  Catskills.  Its  southeasterly  half  is  a  mountain 
ridge  partaking  of  extensive  faulting  and  folding  and  represented 
by  the  Hudson  River  slates  overlain  unconformably  by  the  thick 
and  very  resistant  Shawangunk  conglomerate  forming  high  east- 
ward-facing cliffs.  Toward  the  northwest  these  disturbances  dimin- 
ish, the  strata  gradually  pass  deeper  beneath  a  great  succession  of 
shales,  limestones,  and  sandstones  of  the  Helderbergian  series,  and 
a  broad  valley  is  eroded  in  the  softer  portions.  It  is  limited  on  the 
northwest  by  the  prominent  and  very  persistent  escarpment  border- 
ing the  Hamilton  series  and  forming  the  outer  margin  of  the  Cats- 
kill  mountains. 

e  The  Catskill  area  is  of  simple  structure.  The  strata  are 
well  bedded  and  lie  almost  flat  with  a  gentle  dip  northwest.  The 
surface  features  form  a  series  of  irregularly  distributed  escarp- 
ments, hills,  valleys,  cliffs,  gorges  and  mountains,  rising  rapid!/ 
toward  the  west,  with  moderate  to  strong  relief  and  reaching  ele- 
vations of  2500  feet.  The  failure  of  the  northeast-southwest  trend 
of  feature  that  is  so  common  in  all  of  the  other  districts  is  a  marked 
difference.   It  is  directly  due  to  the  flatness  of  the  strata. 

2  Stratigraphy 

There  are  no  strata  of  prominence  in  association  with  the  main 
aqueduct  younger  than  Devonic  age  except  the  glacial  drift.  Imme- 
diately adjacent  areas,  however,  some  of  which  are  covered  by  the 
accompanying  maps,  and  Long  Island  have  later  formations  ex- 


32 


NEW   VORK  STATE  MUSEUM 


tensively  developed.  Such  are  the  Triassic  rocks  of  the  New  Jersey 
side  of  the  Hudson  below  the  Highlands,  and  the  Cretaceous  and 
Tertiary  strata  of  the  Atlantic  margin  on  Long  Island  and  Staten 
Island.  The  development  of  underground  water  supply  on  Long 
Island  is  especially  concerned  with  these  later  formations,  and  with 
the  modified  drift  deposits  of  the  continental  margin. 

The  whole  series  of  formations  are  more  commonly  considered 
in  groups  that  exhibit  certain  age  or  physical  unity  and  that  are 
for  the  most  part  characteristic  of  certain  regional  belts  and  that 
coincide  somewhat  roughly  with  the  physiographic  divisions  already 
noted.  There  is  in  the  following  description  and  tabulation  no  direct 
attempt  to  unduly  emphasize  this  relation  or  to  belittle  the  divisions 
recognized  in  the  commonly  adopted  geologic  column.  It  is,  how- 
ever, for  the  purpose  in  hand,  more  convenient  and  useful  to  keep 
clear  the  physical  groupings,  because  largely  these  groups,  instead 
of  the  more  arbitrary  subdivisions  of  age,  are  the  units  used  in  con- 
sidering structural  and  applied  problems. 

a  Quaternary  deposits,  (i)  Glacial  drift.  A  loose  mantle  of 
soil  and  mixed  rock  matter  covers  the  bed  rock  throughout  the 
whole  region  except  (a)  here  and  there  where  the  rock  sticks  up 
through  (outcrops),  and  (b)  at  the  most  southerly  margin  along 
the  coast  where  the  glaciers  seem  not  to  have  reached. 

Origin.  This  mantle  is  usually  very  different  in  lithologic  charac- 
ter from  the  underlying  rock  floor.  There  is  almost  always  an 
abrupt  break  between  the  rock  floor  and  the  overlying  material. 
The  rock  floor  is  grooved,  smoothed,  and  scratched  as  if  by  the 
moving  of  rock  or  gravel  over  it.  The  larger  boulders  are  usually 
of  types  of  rock  identical  with  ledges  lying  northward  at  greater  or 
less  distance.  Materials  of  exceedingly  great,  variety  both  in 
size  and  condition  and  lithologic  character  are  often  all  piled  to- 
gether in  the  most  hopelessly  heterogeneous  manner.  These  are 
now  commonly  regarded  as  conclusive  evidence  of  glacial  origin. 
There  is  no  need  of  making  the  discussion  exhaustive.  It  is  almost 
universally  called  the  "  drift." 

Thickness.  The  thickness  of  the  drift  varies  from  almost  o  to  ap- 
proximately 500  feet.  It  is  generally  thickest  in  the  valleys  where  it 
has  simply  filled  many  of  the  original  depressions  and  obliterated 
much  of  the  ruggedness  of  surface,  the  gorges  and  ravines  and  can- 
yons of  the  preglacial  time. 

Sources.  It  appears  from  an  examination  of  the  grooves  and 
striae  on  bed  rock,  and  the  relationship  of  the  different  types  of 
drift  to  each  other,  and  from  a  comparison  of  the  types  of  boulders 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


33 


with  the  ledges  that  may  be  regarded  as  their  source,  that  the  gen- 
eral ice  movement  was  from  north  to  south  swerving  along  the 
southerly  extension  to  east  of  south.  Therefore  it  is  not  unusual 
to  find  abundant  boulders  of  Palisade  trap  stranded  in  New  York 
city  or  on  Long  Island,  or  boulders  of  the  Cortlandt  series,  or  of 
the  gneisses  of  the  Highlands,  or,  in  occasional  instances,  of  sand 
stones  from  the  Catskills,  or  the  limestones  from  the  Helderbergs 
or  perhaps  in  rarer  cases  even  rocks  from  greater  distance,  as  the 
Adirondack  mountains. 

Kinds  of  drift.  There  are  in  the  region  two  fundamentally  differ- 
ent types  of  drift  as  to  method  of  deposition.  They  are  (a)  unas- 
sorted drift  (till  or  hardpan),  and  (b)  modified  drift  (stratified  or 
partially  assorted  gravels,  sands,  clay,  etc.).  The  former  (a)  repre- 
sents deposition  directly  from  the  ice  sheet  at  its  margin  (terminal 
or  marginal  moraines)  or  beneath  ("ground  moraine")  without 
enough  water  action  to  rework  and  assort  the  material.  It  there- 
fore contains  boulders,  pebbles,  sand  and  clay  of  a  heterogeneous 
mixture  of  the  most  complex  sort  both  as  to  size  and  character.  In 
such  deposits  there  is  almost  always  sufficient  intermixture  of  clay 
and  rock  flour  of  the  finest  sort  to  make  a  very  compact  and  dense 
mass  that  is  usually  quite  impervious  to  water.  Such  deposits  are 
distributed  rather  unevenly  over  the  surface  and  where  this  uneven- 
ness  leaves  hollows  or  basins,  or  obstructs  the  outlets  of  other  de- 
pressions, they  may  hold  water  and  form  small  lakes  or  ponds  or 
swamps.  This  is  almost  universally  the  origin  of  the  many  thou- 
sands of  lakes  of  the  northern  lake  region.  It  is  evident  that  ma- 
terial of  this  character,  a  type  that  commonly  serves  the  purpose  of 
a  natural  dam  or  reservoir,  would  be  especially  important  and  useful 
at  certain  places  on  the  Catskill  system.  As  a  matter  of  fact,  so  far 
as  geologic  features  are  concerned,  it  is  the  chief  factor  in  choice  of 
location  for  the  Ashokan  dam  [see  discussion  pt  2]  and  is  a  con- 
trolling factor  in  the  plans  for  the  erection  of  the  miles  of  dikes 
at  less  critical  margins  of  the  reservoirs.  Till  is  an  extensively 
developed  type  but  frequently  passes  abruptly  either  laterally  or 
vertically  into  assorted  materials  of  very  different  physical  char- 
acter. 

(b)  All  materials  associated  in  origin  with  the  glacial  occupation 
that  have  been  materially  modified  especially  in  the  direction  of  an 
assorting  of  material  are  referred  to  as  "  modified  drift  "  deposits. 
They  include  (1)  deposits  made  by  both  water  and  ice  together, 
(2)  those  formed  by  running  water.  (3)  those  laid  down  in  stand- 


34 


NEW   YORK  STATE  MUSEUM 


ing  water.  Or  again  (i )  those  accumulated  rapidly  with  very  irreg- 
ular supply  of  material  at  the  margin  of  the  ice-forming,  hummocky 
or  hill  and  kettle  surface  (kames,  eskers),  (2;  those  carried  along 
valleys  or  general  lines  of  drainage  to  a  considerable  distance  beyond 
the  ice  margin  aggrading  the  valley  with  the  overload  of  gravels 
and  sands  (valley  trains),  (3)  those  washed  out  from  the  ice  margin 
in  more  even  distribution  forming  a  gently  sloping  and  thinning 
extramarginal  fringe  (outwash  or  apron  plains),  (4)  those  fine 
matters  that  are  carried  by  glacial  streams  into  the  margins  of  more 
quiet  waters,  either  a  temporary  or  a  permanent  lake  or  a  larger 
and  slower  stream  or  other  body  forming  more  perfectly  assorted 
and  more  evenly  stratified  deposits  (delta  deposits),  (5)  those 
finer  rock  flours  and  clays  that  remain  suspended  longer  and  carry 
out  much  farther  settling  only  in  the  very  quiet  waters  of  lakes  01 
estuaries  or  temporary  water  bodies  of  this  character  forming  the 
perfectly  banded  clays  (glacial  lacustrine  clays). 

It  is  evident  then  that  modified  drift  has  in  the  process  of  its 
accumulation  suffered  chiefly  a  separation  of  fine  from  the  coarse 
particles  and  that  in  most  cases  the  fine  clay  filling  that  makes  the 
till  dense  and  impervious  to  water,  has  been  washed  out  and  de- 
posited by  itself  in  the  more  inaccessible  deeper  waters.  As  a  re- 
sult most  modified  drift  deposits  are  pervious  and  easy  water 
conductors,  but  poor  or  questionable  ground  for  dikes  or  dams  or 
basins    [sec  discussion  of  Ashokan  dam,  pt  2]. 

Some  of  them,  the  medium  sands  and  gravels,  furnish  an  excel- 
lent and  already  cleaned  structural  material  for  concrete  or  mortar, 
such  as  the  Horton  sand  deposit,  or  coarser  kinds  may  be  crushed 
and  sued  before  using  as  is  done  at  Jones  Point  on  the  Hudson. 

The  finer  silts  and  clays,  usually  overlain  by  assorted  sands,  are 
abundant  along  the  Hudson,  having  been  deposited  there  at  a  time 
when  the  water  of  this  estuary  stood  50  to  150  feet  higher  than 
now.  Recent  erosive  activity  of  the  river  has  cut  the  greater  pro- 
portion of  the  original  deposits  away  but  at  many  places  large  quan- 
tities still  remain  above  water  level  in  the  banks  and  still  greater 
quantities  extend  beneath  the  river.  These  deposits  are  the  support 
of  the  brick  industry  of  southeastern  New  York.  The  till  deposits 
are  very  difficult  to  penetrate  in  making  borings  because  of  the 
boulders,  the  wash  rig  being  almost  useless.  Modified  drift  of  the 
medium  and  finer  sorts  is  easily  and  cheaply  penetrated,  and,  if  it 
lies  on  bed  rock,  such  exploration  gives  reliable  results. 

Structure.  But  this  is  stating  the  actual  conditions  too  simply. 
The  glacial  epoch  was  a  complex  one   The  continental  ice  sheet  may 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


35 


have  advanced  and  retreated  repeatedly,  how  many  times  in  this 
region  is  not  clear.  \\  ith  each  time  of  advance  and  retreat,  the 
work  done  by  it  partly  destroyed,  or  disturbed  or  modified  or  cov- 
ered the  earlier  ones  in  what  appears  now  to  be  a  most  arbitrary 
way  (in  reality,  of  course,  in  a  very  consistent  way  for  the  condi- 
tions that  then  existed).  So  one  frequently  finds  a  till  beneath  a  de- 
posit of  stratified  drift,  or  modified  drift  beneath  till,  or  a  succession 
of  a  still  greater  number  of  changes  in  almost  hopeless  confusion. 
In  New  York  city,  for  example,  at  Manhattanville  cross  valley, 
the  exposed  drift  above  street  level  includes  (a)  at  the  bottom, 
water-marked  stony  till  and  assorted  gravels,  (b )  in  the  middle  per- 
fectly horizontal,  stratified  rock  flour  and  the  finest  sand,  (c)  top, 
wholly  unassorted  bouldery  till,  covered  by  thin  soil.  It  is  evident 
that  the  most  careful  and  accurate  identification  of  the  surface  type 
without  subsurface  investigation  would  give,  for  such  uses  as  are 
now  being  considered,  thoroughly  unreliable  evidence  as  to  the 
behavior  of  the  whole  body  at  this  point.  Therefore,  a  determina- 
tion of  the  changes  and  quality  forms  an  essential  record.  All  of 
these  types  are  to  be  found  in  the  region,  but  the  different  grades  of 
till  and  roughly  modified  material  belonging  to  the  kame  type  are 
more  common  inland. 

On  Long  Island  the  development  of  marginal  modified  types  is 
extensive  and  more  or  less  obscured  by  the  advance  and  retreat 
noted  above.  The  larger  divisions  recognized  in  deposits  are  (a) 
an  early  accumulation  of  sands  and  gravels,  strongly  developed  near 
the  western  end  of  the  island,  known  as  the  "  Jameco "  gravel, 
(b)  an  interglacial  (retreatal)  deposit  of  blue  clays  known  as  the 
"  Sankaty  "  beds,  (c)  a  later  series  of  deposits,  sands,  clays,  gravels 
and  till,  belonging  to  the  closing  stages  of  the  ice  period  correspond- 
ing to  the  surface  deposits  of  the  larger  portion  of  the  whole  region 
(Tisbury  and  Wisconsin  advances).  Some  of  these  sands  and 
gravels  are  important  water-bearing  sources  for  the  new  Brooklyn 
additional  supply. 

The  whole  Long  Island  series  according  to  Veatch1  includes : 


Tisbury  stage  <J 


Wisconsin  stage 


1  After  PP  44,  U.  S.  Geological  Survey,  p.  33. 
2 


36 


NEW  YORK  STATE  MUSEUM 


Gay  Head       I  FoldinS  (glacial  folding) 

\  Sankaty  retreatal  stage  (interglacial)  clay  beds 

Jameco  <f  Glacial  -  Jameco  gravels 

L  rostmannetto  erosion  (interglacial) 

Mannetto  stage  Glacial  —  old  gravels 

A  radically  different  and  in  some  respects  a  much  simpler  inter- 
pretation1 of  the  Long  Island  deposits  has  been  outlined  by  W.  O. 
Crosby.  The  essential  feature  of  his  classification  is  the  unity  and 
simplicity  of  the  glacial  epoch.  Only  the  moraines  and  associated 
sands  and  gravels  of  outwash  origin  during  advance  and  retreat  are 
regarded  as  glacial.  All  other  deposits  below  and  including  the 
Sankaty  clay  beds  he  regards  as  preglacial. 

The  Jameco  gravels  are  interpreted  as  Miocene  in  age. 

Certain  persistent  yellow  gravels  overlying  the  Jameco  are  classi- 
fied as  Pliocene. 

b  Tertiary  and  Cretaceous  deposits.  (2)  Tertiary  outliers. 
Deposits  of  Pliocene  age  are  littoral  in  type  [PP  44  U.  S.  G.  S. 
p.  28]  and  are  not  very  well  differentiated  (Long  Island,  Staten 
Island).    Probably  equivalent  to  the  Bridget  on  beds  of  New  Jersey. 

Certain  "  fluffy  "  sands  in  thin  beds  are  assigned  by  Mr  Veatch 
to  the  Miocene  (Long  Island,  Staten  Island).  Probably  equivalent 
to  the  Beacon  hill  deposits  of  New  Jersey.  Crosby  places  the 
Jameco  gravels  in  the  Miocene  together  with  the  Kirkwood  lignitic 
and  pyritic  clays  and  sands. 

(3)  Upper  Cretaceous  deposity2  are  extensively  developed. 
They  form  the  chief  bed  rock  of  Long  Island. 

1  The  writer  offers  both  of  these  outlineG  of  the  glacial  and  associated 
deposits  in  preference  to  either  alone.  Both  Veatch  and  Crosby  have  given 
immensely  more  time  to  the  study  of  these  questions  than  any  one  else. 
It  is  hardly  fitting  for  a  newcomer  in  their  field  to  reject  either  view.  But 
because  of  the  very  great  difference  between  the  two  interpretations  one 
may  be  pardoned  a  preference.  It  is  the  writer's  opinion  that  the  simpler 
outline  is  the  more  tenable.  It  does  not  seem  possible  to  establish  a  very 
complex  series  of  stages  in  the  glacial  epoch  as  represented  in  the  deposits 
of  southeastern  New  York. 

2  Crosby's  classification  of  the  Cretaceous  is  as  follows : 

(a)  Monmouth  —  slight  development  of  marls.     (Lower  and  middle 

marl  series.) 

(b)  Matawan — (clay  marl  series)  probably  present  on  Long  Island. 

(c)  Magothy  —  an  extensive  series  of  variegated  and  micaceous  sands 

and  clays.    Heavy  development  on  Long  Island. 

(d)  Raritan  —  Plastic  clay  scales  and  the  Lloyd  sand. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


37 


(o)  A  lignitiferous  sand  with  occasional  clay  beds  forming  the 
uppermost  of  the  Cretaceous  series  is  probably  equivalent  to  the 
marl  series  of  New  Jersey.  But  it  lacks  the  prominent  greens  and 
development  characteristic  of  the  region  further  south.  Not  clearly 
separable  from  the  underlying  formation  or  Matawan  beds. 

(£?)  The  Matawan  beds.    Gray  sands  and  clays. 

(c)  Raritan  formation.  Clays  and  sands,  plastic  clays,  the  Lloyd 
sand,  an  important  water  carrier  lies  about  200  feet  below  the  top 
of  the  formation.   Occasional  leaf  impressions. 

All  of  these  formations,  except  where  disturbed  locally  by  glacial 
ice,  dip  gently  seaward.  The  sand  beds  of  these  strata  are  the  chief 
sources  of  underground  water  being  developed  by  the  new  system. 

c  Jura-Trias  formations.  (4)  Palisade  diabase.  This  is  a  thick 
intrusive  sheet,  or  sill,  of  igneous  rock  of  diabasic  type.  It  is 
700-1000  feet  thick.  It  lies  for  the  most  part  parallel  to  the  bed- 
ding of  the  surrounding,  inclosing,  sedimentary  rocks,  and,  rising 
gently  eastward,  forms  a  strong  cliff  continuously  along  the  west 
bank  of  the  Hudson  for  40  miles.  It  varies  from  very  fine  to  very 
coarse  texture  and  is  for  the  most  part  fresh,  tough,  durable,  and 
is  the  source  of  large  quantities  of  the  most  satisfactory  quality  of 
crushed  stone  now  on  the  market  for  use  in  concrete. 

(5)  Newark  scries.  This  is  a  very  great  thickness  of  silicious 
sediments,  chiefly  reddish  conglomerates,  red  and  brown  quartzose 
and  feldspathic  sandstones  and  shales.  They  dip  gently  westward 
and  northwestward  at  10-20  degrees,  and  are  confined,  in  this 
region,  to  the  west  side  of  the  Hudson  south  of  the  Highlands.  The 
formation  supplies  "  brownstone  "  for  building  purposes. 

None  of  the  Jura-Trias  rocks,  so  far  as  known,  will  be  cut  by  the 
aqueduct. 

d  Devonic  strata.  (6)  Cat  skill  formation.  This  formation1  is  of 
continental  type,  chiefly  a  conglomerate.  A  white  conglomeratic 
sandstone  forming  the  uppermost  portion  attains  its  greatest  thick- 
ness on  Slide  mountain  (350  feet).  It  is  a  "  coarse  grained,  heavy 
bedded,  moderately  hard  sandstone  containing  disseminated  pebbles 
of  quartz  or  light  colored  quartzite,  and  streaks  of  conglomerate." 

A  red  conglomeratic  sandstone  constitutes  the  much  thicker  por- 
tion below  (1375  feet).  It  is  a  "  coarse,  heavy  bedded  sandstone  of 
dull  brownish  hue  containing  disseminated  pebbles  and  conglom- 
eratic streaks,  differing  from  the  overlying  beds  chiefly  in  color.  In 

1  Grabau,  A.  W.  N.  Y.  State  Mus.  Bui.  92.  Geology  and  Paleontology 
of  the  Schoharie  Valley. 


38 


NEW   YORK  STATE  MUSEUM 


both  series  the  pebbles  and  conglomeratic  streaks  are  scattered  and 
irregular,  while  the  sands  are  often  cross-bedded.  Thin  layers  of 
red  shale  occur,  and  locally  gray  sandstones."  The  deposits  prob- 
ably represent  Hood  plains,  deltas,  and  alluvial  fans  accumulated 
mostly  above  sea  level. 

(7)  Onconta  sandstone  (Upper  flagstone).  "Thin  and  thick 
bedded  sandstones  from  20  to  200  feet  thick  with  interbedded  red 
shales  up  to  30  feet  thick."  Chiefly  light  gray  to  brown  in  color. 
Abundant  cross-bedding,  occasional  dark  shale,  frequent  flagstone 
beds.  Capable  of  furnishing  "  bluestone  "  flags  and  more  massive 
dimension  stone.  To  be  seen  in  the  vicinity  of  West  Shokan  and 
westward. 

(8)  Ithaca  and  Sherburne  (lower  flagstone  "  bluestone  ").  "  Thin 
bedded  sandstone,  with  intercalated  beds  of  dark  shale.  The  sand- 
stones are  in  masses  from  a  few  inches  to  40  feet  in  thickness, 
greenish  gray  to  light  bluish  gray  or  dark  gray  in  color,  and  are 
extensively  quarried  as  flagstones."  There  are  occasional  conglom- 
eratic streaks.  Occurs  in  large  development  in  the  vicinity  of  the 
Ashokan  reservoir  (500  feet).  The  heavier  cross-bedded  and 
coarser  grained  beds  are  capable  of  furnishing  an  unusually  good 
quality  of  large  dimension  stone  for  heavy  structural  uses.  The 
beds  of  this  formation  near  Olive  Bridge  will  in  all  probability 
furnish  the  greater  proportion  of  stone  of  all  kinds  for  the  con- 
struction of  the  great  Ashokan  dam  [see  discussion  of  bluestone 
near  Ashokan  dam,  pt  2].  The  chief  common  fossil  content  is 
impressions  of  plant  remains. 

(9)  Hamilton  and  Marccllus  shales.  "  Dark  gray  to  black  or 
brown  shales  with  thin  arenaceous  beds  in  the  upper  part."  Forms 
the  upper  portion  of  the  escarpment  that  follows  the  outer  margin 
of  the  Catskill  foothills  bordering  the  westerly  side  of  the  middle 
Rondout  and  lower  Esopus  valleys.  Occasionally  beds  are  sub- 
stantial enough  for  flagstone  production  (700  feet  or  more  with  the 
Marcellus.) 

The  chief  index  fossils  are:  Spirifer  mucronatus, 
Athyris     spiriferoides,     Chonetes  coronatus. 

The  Marcellus  shale  is  not  readily  differentiated  in  the  Esopus 
valley  Characteristically  it  is  a  thin  bedded  shale  of  no  great 
thickness  (180  feet  in  the  Schoharie  valley)  lying  between  the 
Onondaga  limestone  and  the  Hamilton  and  obscured  by  talus  from 
the  escarpment  (with  the  Hamilton  700  feet.) 

Styliolina  fissurella,  Chonetes  mucronatus, 
Strophalosia  truncata,   Liorhynchus  mysia. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


39 


The  dividing  lines  between  the  different  sandstones  and  shale 
formations,  the  Oneonta,  Ithaca,  Sherburne,  Hamilton  and  Mar- 
cellus, can  not  be  sharply  drawn  in  the  Ksopus  region.  Together 
they  form  in  a  large  way  a  rather  satisfactory  held  unit.  For 
specific  purposes  it  is  necessary  to  recognize  that  the  lower  por- 
tions are  prevailingly  shales  with  thin  bedded  sandstones  while  the 
upper  portions  are  much  more  heavily  bedded,  the  sandstones  pre- 


•••  •  v    .  - 


Fig.  2    Spirifer  mucron  atus    (Conrad),   a  characteristic  and  abundant  index 
fossil  of  the  Hamilton  shales  of  the  Cat.skiil  margin 

vailing.  The  five  divisions  may  possibly  be  more  satisfactorily 
made  on  paleontologic  characters  than  on  physical,  but  in  most  of 
the  advisory  reports  on  economic  and  practical  problems  involving 
this  district  the  subdivisions  can  not  be  emphasized.  The  wdiole 
scries  is  essentially  conformable  and  is  very  little  disturbed  [see 
report  on  Milestone  quarries,  pt  2]. 

(10)  Onondaga  limestone.  A  bluish  gray,  massive,  thick  bedded 
cherty,  somewhat  crystalline  limestone.  It  is  strongly  marked  off 
from  the  Hamilton  and  Marcellus  above,  and,  because  of  its  greater 
resistance  to  erosion,  usually  forms  a  dip  slope  controlling  stream 
adjustment  and  ultimately  inducing  the  development  of  unsymmet- 
rical  valleys  w  ith  gentle  easterly  slopes  and  clifflike  westerly  borders 
where  the  streams  are  sapping  the  overlying  Marcellus  and  Ham- 
ilton shales.  It  is  not  sharply  separable  from  the  Esopus  below  but 
everywhere  in  this  region  graduates  into  it  with  increase  of  silicious 


40 


NEW  YORK  STATE  MUSEUM 


and  argillaceous  impurities.  Estimating  the  formation  from  the  drill 
cores  that  have  penetrated  it,  and  placing  the  lower  limit  as  nearly 
as  may  be  at  the  horizon  of  changes  from  predominant  lime  to  pre- 
dominant silicious  content,  the  approximate  thickness  in  this  region 
is  placed  at  200  feet.  The  rock  where  exposed  exhibits  considera- 
ble joint  development  and  these  are  considerably  enlarged  by  the 
solvent  action  of  percolating  waters.  This  factor  is  considered  of 
some  importance  in  connection  with  the  other  limestones  of  the 
district  in  aqueduct  construction  and  permanence.  The  Onondaga 
has  been  used  as  a  building  stone  formerly  sold  as  marble,  some 
grades  of  which  are  good  stone.  On  the  line  of  the  aqueduct  it  is 
confined  to  the  Rondout  and  Esopus  valleys.  The  chief  fossils  are: 
Atrypa  reticularis,  Zaphrentis  prolifica, 
Leptostrophia  perplana,  Platyceras  dumosum, 
Leptaena  rhomboidalis,  Dalmanites  selenurus. 

(11)  Esopus  and  Schoharie  shales  (a  slaty  grit).  The  Schoharie 
as  a  distinct  formation  is  not  distinguishable  in  this  region.  The 
very  thick  and  comparatively  uniform,  gritty,  black,  dense,  almost 
structureless  rock  is  a  distinct  unit.  It  is  a  silicious  mud  rock  with 
very  obscure  sedimentation  markings,  but  showing  independent 
secondary  cleavages  induced  by  later  dynamic  factors,  and,  on  long 
exposed  surfaces  always  exhibiting  chiplike  fragments  as  the  result 
of  weathering.  But  it  is  not  an  easily  destroyed  rock.  In  so  far 
as  the  bedding  is  obscure  and  the  induced  structure  predominates, 
the  rock  is  a  slate;  and  in  so  far  as  it  is  distinctly  gritty  (sandy) 
instead  of  argillaceous  it  is  a  grit.  The  formation  might  therefore 
be  more  accurately  designated  as  a  slaty  grit.  The  lack  of  plain 
bedding  structure  makes  it  impossible  to  estimate  its  thickness, 
since  the  foldings  or  other  displacements  can  not  be  allowed  for ; 
but  the  accumulated  data  of  drill  holes  in  more  advantageous 
position  indicate  an  approximate  thickness  of  800  feet.  The  rock 
is  considered  exceptionally  good  ground  for  the  tunnel. 

A  few  fossils  occur  the  most  characteristic  being  Taonurus 
c  a  u  d  a  g  a  1 1  i .  There  are  also  in  certain  layers  of  limited  extent, 
Leptocoelia    acutiplicata  and  Atrypa  spinosa. 

(12)  Oriskany  and  Port  Eivcn  transition  (silicious  shaly  lime- 
stone). There  is  no  well  defined  and  distinct  separation  here  be- 
tween the  Oriskany  and  the  underlying  Port  Ewen,  but  because  of 
the  importance  and  persistence  of  the  formation  in  other  and  re- 
lated areas  the  name  is  held.  The  equivalent  of  the  Oriskany  is  in 
this  district  involved  with  a  strongly  developed  transition  zone 
which  in  physical  features  is  intimately  associated  with  the  Port 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


41 


Ewen  as  a  single  unit.  If  any  distinct  formation  is  to  be  recog- 
nized it  would  be  on  the  basis  of  transitional  faunal  character, 
placing  the  fossiliferous  upper  100  feet  in  the  Oriskany  transition 
and  confining  the  name  Port  Ewen  to  the  rather  unfossiliferous 
and  concretionary,  shaly,  argillaceous  limestone  of  the  lower  100 
feet. 


Fig.  3    Spirifer  arenosus  (Conrad),  one  of   the  characteristic  index  fossils  of 
the  Oriskany  occurring  in  the  Port  Ewen-Oriskany  transition 


This  transition  rock  is  strongly  bedded,  argillaceous  and 
silicious  limestone,  very  quartzose  in  certain  layers,  but  there 
are  no  exposures  in  this  area  that  would  be  called  sandstones. 
Fossils  are  abundant  and  show  marked  Oriskany  peculiarities. 
Those  of  most  characteristic  relations  are :  Hipparionyx 
proximus,  Leptostrophia  magnifica,  Spirifer 
murchisoni,  Spirifer  arenosus,  Platyceras 
nodosum,    Strop  hostylus  expansus. 


42 


NEW  YORK  STATE  MUSEUM 


(13)  Port  Ewen  shaly  limestone.  The  beds  below  those  noted 
in  the  preceding  paragraph  are  essentially  argillaceous,  shaly  lime- 
stones. They  vary  from  rather  massive  to  thin  bedded,  are  dark 
grayish  in  color,  and  have  a  peculiar  nodular  or  concretionary  de- 
velopment along  certain  sedimentation  lines.  These  spots  have  less 
resistance  to  weather  than  the  surrounding  rock  and  therefore 
develop  rows  of  pits  along  the  face  of  an  outcrop.  Their  size, 
6  to  18  inches  or  more  across,  together  with  their  persistence  makes 
an  easily  recognized  physical  feature.  The  few  fossils  that  are 
found  are  not  very  characteristic.  The  following  should  be  men- 
tioned :   Spirif er  perlamellosus. 

In  the  discussion  and  on  the  maps  the  Port  Ewen  and  Oriskany 
are  treated  together  as  a  single  unit  as  the  Oriskany-Port  Ewen 
beds. 

(14)  Becraft  limestone.  Massive,  heavy  to  thin  bedded,  light 
colored,  semicrvstalline  to  thoroughly  crystalline  limestone.  More 
massive  beds  very  pure,  94  -1-  i  Cat  ( ) ...    Shaly  beds  resemble  the 

New  Scotland  which  they  pass 
into  at  the  base.  The  most  char- 
acteristic features  for  field  iden- 
tification are  (a)  pink  or  light 
colored  spots,  (£>)  a  more 
coarsclv  crvstal'ine  condition 
than  any  of  the  associated  strata, 
(c)  occasional  large  calcite 
cleavages  to  be  seen  wherever 
a  fossil  crinoid  base  A  s  p  i  d  o  - 

Fig.  4    Sieberella  pseudogal-  CrillUS  SCUtelliformis 

e  a  t  a    Hall,  the  most   characteristic  index  •     i       i  /  j\    .1  i 

fossil  of   the  Beacroft  limestone  of  the  Ron-  IS  broken,  (0 )    the  Very  CharaC- 

dout  region  •  -  .  ,.  ,       ..       c  .     ,  .  , 

tenstic     fossil     b  1  ebe  rel  1  a 
pseudogaleata,  and  (e)  many  crinoid  stems. 

The  formation  carries  many,  fossils  in. addition  to  these  given 
above,  among  which  are  Spirifer  concinnus,  Uncin- 
u  1  u  s  campbellanus. 

(15)  A  ezv  Scotland  slialy  limestone.  Thin  bedded,  dark  gray  to 
reddish  sandy  and  shaly  limestones.  The  rock  breaks  out  in  slabs 
on  weathering  and  develops  red  iron  stains.  It  has  especially 
abundant  fossils,  the  most  characteristic  of  which  are:  Ortho- 
t  h  e  t  e  s  w  o  o  1  w  o  r  t  h  a  n  u  s  ,  Spirifer  macropleura. 
Other  common  ones  are :  L  e  p  t  a  e  n  a  r  h  o  m  b  o  i  d  a  1  i  s  , 
Strop  honella  headleyana,  Ripidomella  oblata, 
Strop heodonta  becki. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


43 


(16)  Coeymans  Hun-stone.  Heavy  bedded,  dark  gray,  argillace- 
ous and  flinty  limestone.  The  characteristic  features  for  field 
identification  arc  (a)  abundant  chert  nodules,  (b)- the  occurrence  of 
coral  reef  structure  and  heads  of  corals,  Favosites  h  elder- 


Fig,  s     Spirifer  macropleura  (Conrad),  the  most  characteristic  index  fossil 
of  i he  New  Scotland  beds  in  the  Rondout  region 


b  e  r  g  i  a  .  The  brachiopods  S  i  e  b  e  r  e  1 1  a  g  a  1  e  a  t  a  and 
A  t  r  y  p  a    reticularis    are  very  common. 

This  formation  has  a  thickness  of  about  8o  feet  and  is  rather 
distinctly  separated  from  the  underlying  Manlius.  The  Coeymans 
is  considered  the  base  of  the  Devonic  system  of  New  York.    It  is 


Fig.  6     Sieberella  galeata  (Dalman).  the  most  reliable  index  fossil  of  the 
Coeymans  limestone  of  the  Rondout  region 

perfectly  conformable  upon  the  underlying  series  and  it  is  evident 
that  in  this  region  there  was  no  important  break  in  the  progress  of 
deposition. 

c  Siluric  strata.  (17)  Manlius  limestone.  Lime  mud  rock,  fine 
textured,  dense,  with  plainly  marked  sedimentation  lines,  gray  to 
dark  gray  color.  The  most  characteristic  features  in  the  field  are 
(a)  fine  texture,  (£>)  sedimentation  lines,  as  if  laid  down  in  quiet 
waters  as  a  lime  mud,  (c)  solution  joints  sometimes  enlarged  to 


44 


NEW  YORK  STATE  MUSEUM 


cavelike  form  into  which  surface  streams  disappear  (such  as  Pom- 
pey's  cave  near  High  Falls),  (d)  mud  crack  surfaces  (in  lower 
beds),  (e)  occurrence  of  the  fossil  Leperditia  alta. 

Its  abundant  jointing  and  the  tendency  to  develop  solution  cav- 
ities from  them  is  considered  an  objectionable  character. 

(18)  Cobleskill  and  cement  beds  (limestone).  It  is  not  pos- 
sible without  the  most  painstaking,  comparative,  chemical  and  pale- 
ontologic  research  to  differentiate  the  cement  layers  from  the 
inclosing  beds  and  to  assign  them  all  to  the  subdivisions  that  are 
recognized  in  some  previous  publications,1  as  the  (a)  Rondout 
cement  (b)  Cobleskill  limestone,  (c)  Rosendale  cement,  and  (d) 
Wilbur  limestone.  There  are,  however,  two  workable  natudal  ce- 
ment beds,  both  at  Rondout  and  at  Rosendale,  with  a  nonworkable 
layer  between  each  case,  and  also  one  between  the  lower 
and  the  next  underlying  formation.  Whether  the  two  cement  beds 
at  Rondout  represent  the  Rondout  and  the  Rosendale  horizons 
with  the  Cobleskill  between,  or  whether  they  should  both  be  re- 
garded as  Rondout  with  Cobleskill  below,  can  not  concern  our 
present  problems.  And  again,  whether  or  not  the  two  cement  beds 
at  Rondout  are  the  same  two  that  appear  at  Rosendale,  or  whether 
they  are  equivalent  only  to  the  upper  one  with  a  new  lower  bed 
(The  Rosendale)  added  in  this  area  and  then  with  the  Cobleskill 
between  these  two  as  claimed  by  Grabau,  does  not  alter  the  plain 
fact  that  the  whole  series  is  a  physical  unit.  It  is  a  gray,  rather 
close  texture  limestone,  resembling  the  Manlius  proper,  and  con- 
tains few  fossils.  It  is  perhaps  even  better  yet  to  group  all  of 
these  limestone  beds  below  the  Coeymans  into  a  single  unit  and 
call  it  the  Manlius  series. 

(19)  Binnezvater  sandstone.  Below  the  Manlius  cement  rock 
series  lies  the  60-100  foot  Binnewater.  It  is  chiefly  a  well  bedded 
quartz  sandstone,  almost  a  quartzite  in  the  upper  beds  with  more 
shale  in  its  lower  portion,  in  color  varying  from  white  to  greenish 
yellow  and  brown.  The  rock  is  rather  porous  in  certain  beds  and 
especially  along  the  bedding  planes  and  is  not  well  recemented 
where  crushed  by  crustal  movements.  It  is  confined  to  the  Rondout 
valley. 

(20)  High  Falls  shale.2  Greenish  to  red  argillaceous  to  sandy 
shales.    The  exposures  are  often  a  brilliant  red  while  the  rock 

1  N.  Y.  State  Mus.  Bui.  92  (Grabau).  p.  311-13;  N.  Y.  State  Mus.  Bui.  80 
(Hartnagel),  p.  355-58;  N.  Y.  State  Mus.  Bui.  69  (Van  Ingen  and  Clark), 
p.  1 184,  1 185. 

2  The  term  given  by  Hartriagle.    N.  Y.  State  Mus.  Bui.  80.  p.  345. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


45 


from  drill  cores  is  seldom  highly  colored.  The  protected  beds  are 
more  commonly  greenish  in  color  and  contain  much  iron  sulphide. 
Occasional  thin  limestone  beds  occur  in  the  upper  portion  at  High 
falls  —  one  of  4  feet  forms  the  lip  of  the  lower  fall.  The  High 
Falls  shale  is  confined  to  the  Rondout  valley  and  on  the  line  of 
the  aqueduct  is  67-100  feet  thick. 

(21)  Shawangunk  conglomerate.  The  Shawangunk  is  a  con- 
glomerate and  sandstone.  The  constituent  pebbles  are  almost  wholly 
quartz,  well  worn,  and  varying  in  size  from  that  of  sand  to  pebbles 
of  several  inches  diameter.  But  for  the  most  part  the  pebbles  are 
small,  abundantly  mixed  with  sand,  bound  together  by  a  silicious  ce- 
ment. Rarely  a  true  quartzite  is  developed  and  still  more  rarely  a 
shaly  facies.  The  rock  is  therefore  very  hard,  brittle,  and  in  the  un- 
disturbed portions  fairly  impervious  and  resistant.  But  it  suffers 
from  crushing  along  zones  of  disturbance  in  folding  and  faulting 
and  these  zones  are  very  imperfectly  recemented.  It  is  a  durable 
rock,  very  resistant  to  ordinary  decay,  but  forms  great  talus  slopes. 
It  is  used  for  buhrstones  (millstones),  etc.  It  varies  in  thickness 
on  the  lines  of  the  aqueduct  from  280-400  feet.  The  rock  is  lim- 
ited in  its  northward  extension  to  this  district  —  southwestward 
it  is  much  more  broadly  exposed  in  the  continuation  of  the  Shaw- 
angunk range. 

The  Shawangunk  completes  the  conformable  Siluro-Devonic 
series  down  to  the  erosion  interval  at  the  close  of  the  Ordovicic. 
The  series  of  conglomerates,  sandstones,  limestones,  and  shales 
make  an  imposing  column  approximating  3000  feet  of  strata  differ- 
entiated with  more  or  less  ease  into  15  separate  and  mapable 
formations  and  a  possible  5  or  6  more  with  careful  paleontologic 
work.  The  series  begins  with  the  capping  beds  of  the  Shawangunk 
range  and  its  northward  extension  toward  the  Hudson  river  at 
Rondout  and  Kingston,  and  thence  westward  constitutes  the  rock 
floor  while  its  structures  control  the  surface  configurations  far  be- 
yond the  limits  of  the  region  under  consideration.  Immediately 
to  the  north  and  partly  within  the  area  here  treated  is  the  famous 
Rosendale  cement  region,  the  pioneer  cement  district  of  America 
and  for  many  years  the  best  producer.  The  strata  used 
are  almost  exclusively  the  upper  members  of  the  Siluric 
{"  cement  beds ")  closely  associated  with  the  Cobleskill  between 
the  Manlius  proper  and  the  Binnewater  sandstone.  Rarely  the  Be- 
craft  from  the  Devonic  series  furnishes  some  cement  rock. 

/  Cambro-Ordovicic  formations.  Between  the  Precambric 
metamorphics  of  the  Highlands  beneath  and  the  Siluro-Devonic 


46 


NEW  YORK  STATE  MUSEUM 


sediments  of  the  Shawangunk  range  and  the  Catskills  above,  lies 
a  series  of  quartzites,  limestones  and  slates  less  complexly  dis- 
turbed than  the  older  and  more  disturbed  than  the  younger  series 
—  set  off  from  both  by  unconformities  representing  time  intervals 
that  cover  both  folding  and  erosion.  They  are  of  more  than  4000 
feet  thickness  —  how  much  more  it  is  impossible  to  estimate  be- 
cause of  the  obscurity  of  data  in  the  slates.  There  are  very  few 
fossil  forms  preserved  in  them.  The  series  is,  however,  readily 
and  sharply  separable  into  three  formations  that  may  be  mapped 
upon  lithologic  characters  alone.  They  are  of  most  importance  in 
the  Wallkill  valley,  Moodna  creek,  Newburgh,  Fishkill,  New  Ham- 
burg and  Poughkeepsie  districts.  Their  character,  structure,  and 
conditions  have  required  careful  consideration  in  the  decisions  on 
the  Wallkill  and  Moodna  siphons  and  in  the  discussions  on  the 
proposed  Hudson  river  crossings  [sec  Hudson  river  crossings, 
pt  2]. 

(22)  Hudson  River  slates.  The  upper  member  of  the  Cambro- 
Ordovicic  series  is  in  itself  complex.  Prevailingly  it  is  a  slaty 
shale,  occasionally  it  is  a  sandstone  or  shaly  sandstone,  or  a  simple 
shale ;  still  more  rarely  it  is  almost  a  true  slate,  and  very  rarely 
a  phyllite.  The  constituents  vary  from  prevailing  clay  to  quartz 
sand  repeatedly  in  almost  every  locality.  It  is  probable  that  as  a 
rule  the  upper  portions  are  the  more  heavily  bedded  and  arena- 
ceous. The  rock  is  excessively  affected  by  the  dynamic  movements 
that  have  at  least  twice  disturbed  it.  A  slaty  cleavage  in  the  more 
argillaceous  members  is  most  noticeable,  but  almost  everywhere  the 
strata  are  strongly  tilted,  crumpled,  broken,  faulted,  or  crushed  in  a 
most  confusing  way.  This  together  with  an  original  obscurity 
in  bedding,  and  the  obliteration  by  subsequent  shearing  of  much 
that  did  exist,  makes  it  impossible  to  reconstruct  the  complicated 
structure  or  compute  the  thickness  of  the  formation.  It  is  of  such 
physical  character  as  to  absorb  within  its  own  limits  much  of  the 
disturbing  movements,  and  neither  the  formations  above  nor  imme- 
diately below  are  so  extensively  and  intimately  affected.  The 
formation  is  widely  exposed  and  forms  the  bed  rock  over  very  large 
areas.  Almost  everywhere  it  is  impervious  to  water,  easy  to  pene- 
trate by  drill  or  tunnel,  and  resistant  to  decay.  A  few  Ordovicic 
fossils  may  be  found,  the  most  characteristic  being  D  a  1  m  a  n  e  1 1  a 
testudinaria. 

(23)  Wappingcr  limestone}     (In  part  Cambric,  and  in  part 

1  The  Wappinger  Valley  limestone  of  Dwight  (1879)  and  Dana.  The 
Wappinger  limestone  of  Darton  and  others. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


47 


Ordovicic).  The  formation  is  prevailingly  of  a  compact,  fine 
texture,  dark  gray,  either  massive  or  strongly  bedded  limestone. 
Where  the  stratification  is  very  plain  there  are  light  and  dark  layers 
and  an  abundant  silicious  intermixture.  In  many  outcrops  the  rock 
is  so  massive  that  even  the  dip  and  strike  are  obscure.  Some  places 
the  rock  is  fine  crystalline,  almost  a  micromarble.  On  weathered 
surfaces  it  almost  always  exhibits  a  crisscross  etching  which  marks 
the  traces  of  rehealed  cracks.  From  these  it  is  seen  that  many  of 
the  apparently  massive  compact  beds  have  at  one  time  been  exten- 
sively crushed.  In  many  places  there  is  scarcely  a  square  inch 
wholly  free  from  these  evidences.  The  formation  is  best  exposed 
in  the  wide  belt  that  extends  southwestward  from  the  vicinity  of 
Poughkeepsie  and  crosses  the  Hudson  at  New  Hamburg  into  the 
Newburgh  district.  It  undoubtedly  underlies  the  slates  in  the  rest 
of  the  adjacent  area.  There  are  few  fossils  and  they  are  rarely 
found. 

(24)  Poughquag  quartsite.  ■  Below  the  Wappinger  limestone  and 
upon  the  upturned  and  eroded  edges  of  the  Highlands  gneisses  lies 
a  quartzite  of  variable  thickness  but  which  reaches  at  least  600  feet. 
It  is  a  strongly  silicified  quartz  sandstone  —  a  quartzite  by  indura- 
tion. It  is  strongly  bedded  but  seldom  shaly.  Traces  of  schistosity 
may  appear  in  certain  zones  and  this  is  somewhat  strongly  developed 
outside  of  the  area  at  the  type  locality  (Poughquag,  N.  Y.). 

Only  fragments  of  trilobite  spines  have  been  found  in  this  forma- 
tion within  the  district. 

g  Later  crystallines  south  of  the  Highlands.  South  of  the 
Highlands  proper  except  at  one  locality  (Peekskill  creek  valley  and 
its  southwestward  continuation  through  Tompkins  Cove  and  Stony 
Point)  the  rocks  are  all  much  more  thoroughly  crystalline.  There 
are  two  formations,  and  in  places  traces  of  a  third,  above  the  Gren- 
ville  gneisses  (Fordham  gneisses  and  associates).  These  are  known 
locally  as  Manhattan  schist,  Inwood  limestone,  and  Lozvcrrc  quartz- 
ite. In  Westchester  and  New  York  counties  the  quartzite 
is  rarely  found,  and  in  a  considerable  proportion  of  those  places 
where  it  does  occur  its  relations  are  more  consistent  with  the 
gneisses  below  than  with  the  limestone-schist  series  above.  This 
is  true  indeed  of  the  type  locality  (Lowerre).  There  are,  however, 
at  least  two  points  where  the  occurrence  favors  the  reverse  inter- 
pretation, so  far  as  any  is  shown,  and  therefore  a  quartzite  may  be 
regarded  as  finishing  the  series,  and  making  uncertain  but  probably 
unconformable  contact  with  the  underlying  gneisses. 


48 


NEW  YORK  STATE  MUSEUM 


This  series  together  with  the  gneisses  below  constitutes  the  bed 
rock  and  controls  the  underground  conditions  for  all  of  the  line 
south  of  the  Moodna  valley,  50  miles  above  New  York.  All  of  the 
southern  aqueduct,  and  the  New  York  city  distribution  conduits  are 
wholly  concerned  with  these  rocks,  and  two  divisions  of  the  northern 
aqueduct  have  a  large  proportion  of  their  work  in  them. 

It  is  not  wholly  clear  what  age  these  crystallines  represent.  It 
is  certain  that  the  underlying  gneisses  are  Grenville  and  that  the 
metamorphic  quartzite,  Inwood,  Manhattan  series,  is  Post-gren- 
ville.  It  is  possible  that  these  latter  are  also  Precambric.  But 
usage  following  the  correlations  of  Dana1  and  in  the  absence  of 
as  good  evidence  from  any  other  source  has  regarded  them  as  the 
Cambro-Ordovicic  crystalline  equivalents  of  the  Poughquag-YYap- 
pinger-Hudson  River  series  of  the  north  side  of  the  Highlands. 
The  writer  has  elsewhere  shown2  that  the  evidence  and  arguments 
are  not  all  on  one  side  and  that  considerable  doubt  may  still  be 
entertained  on  that  point.  There  is  no  object  in  following  that 
argument  here  or  in  modifying  the  treatment  here  followed  of  mak- 
ing them  a  distinct  series.  Even  if  they  should  prove  to  be  the 
exact  equivalents  of  the  Hudson  River- Wappinger-Poughquag 
series  the  formations  are  physically  so  different  and  require  so 
different  treatment  in  discussion  that  they  must  for  our  present 
purpose  be  regarded  as  an  essentially  distinct  series.  From  that 
standpoint  alone  the  usage  here  followed  is  justified.  The  Man- 
hattan schist  of  Westchester  county  as  a  type  differs  as  much 
petrographically  from  the  Hudson  River  formation  of  the  New- 
burgh  district  as  the  Catskill  formation  of  Slide  mountain  differs 
from  the  Jameco  gravels  of  Long  Island.  In  a  discussion  where 
physical  or  petrographic  character  is  in  control  there  is  no  doubt 
about  the  advisability  of  treating  the  two  separately. 

(1)  Manhattan  schist*  This  is  primarily  a  recrystallized  sedi- 
ment of  silicious  type.  It  occurs  as  a  nearly  black  or  streaked, 
micaceous,  coarsely  crystalline,  strongly  foliated  rock.  The  chief 
constituents  are  biotite,  muscovite  and  quartz.    Quartz,  feldspar, 

1  Dana,  J.  D.  On  the  Geological  Relations  of  the  Limestone  belts  of 
Westchester  county,  N.  Y.  Am.  Jour.  Sci.  20:21-32,  194-220,  359-75,  450-56 
(1880);  21:425-43;  22:103-19,  313-15.  327-35  (1881). 

2  Berkey,  Charles  P.  "  Structural  and  Stratigraphic  Features  of  the 
Basal  Gneisses  of  the  Highlands."  N.  Y.  State  Mus.  Bui.  107  (1907), 
p.  361-78. 

3  Manhattan  schist  of  Merrill.  N.  Y.  State  Mus.  50th  An.  Rep't,  1:287. 
Same  as  "  Hudson  schist,"  of  N.  Y.  city  folio  no.  83. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


49 


garnet,  fibrolite  and  epidote  also  occur  in  large  quantity.  Occa- 
sional streaks  or  masses  are  hornblendic  instead  of  micaceous. 
These  are  interpreted  as  igneous  injections.  They  are  especially 
abundant  on  Croton  lake  and  near  White  Plains. 

It  is  essentially  a  quartz-mica  schist.  But  it  is  almost  everywhere 
very  coarse  textured  and  hardly  ever  exhibits  the  fine  grained,  uni- 
form structure  of  typical  schist.  Its  abnormal  make-up  —  the  pre- 
dominance of  biotite  and  quartz  —  is  the  best  defense  for  its  petro- 
graphic  classification.  The  abundance  of  mica  makes  it  a  tough  rock 
but  not  very  hard.  The  joints  and  fractures  formed  in  later  move- 
ments are  not  healed  and  zones  of  bad  shattering  are  susceptible  to 
considerable  decay.  These  crushings  are  sufficiently  common  to  en- 
courage borings  to  tap  their  content  of  water  for  small  family  use 
throughout  Westchester  county ;  but  they  do  not  represent  large 
circulation  in  any  case.  On  the  whole,  the  rock  if  fresh  is  good 
and  durable.  It  may,  though  rarely,  carry  considerable  sulphide. 
Practically  all  of  the  strictly  original  sedimentation  marks  are  de- 
stroyed by  metamorphism.  The  formation  has  great  thickness,  but 
because  of  the  destruction  of  original  bedding  lines  by  recrystalli- 
zation  and  additional  complication  by  most  complex  folding,  shear- 
ing, crushing  and  faulting,  the  structure  can  not  fully  be  unraveled 
and  the  thickness  can  not  be  estimated  with  any  approach  to  ac- 
curacy of  detail.  But  there  is  probably  a  thickness  represented  of 
several  thousand  feet. 

(2)  Inwood  limestone  or  dolomite.  This  formation  lies  beneath 
the  Manhattan.  It  is  everywhere  coarsely  crystalline  either  massive 
or  strongly  bedded,  often  very  impure  with  development  of  second- 
ary (recrystallized)  mica  (phlogopite)  and  other  silicates,  espe- 
cially tremolite.  It  is  essentially  a  magnesian  limestone  or  dolomite 
in  composition.  There  is  an  occasional  quartzose  bed  in  the  midst 
of  the  limestone  as  at  East  View.  The  upper  beds  are  most  charged 
with  mica  and  occasionally  beds  attacked  by  alteration  have  much 
green,  flaky  chlorite.  There  are  occasional  interbeddings  of  lime- 
stone and  schist  as  a  transition  fades. 

The  coarser  grades  upon  exposure  to  weathering  readily  yield  by 
disintegration  to  a  lime  (calcite)  sand  resembling  rouehlv  an  ordi- 
nary sand  in  general  appearance.  At  Inwood,  the  type  locality,  this 
disintegration  is  so  pronounced  that  great  quantities  are  readily 
shoveled  up  and  used  for  various  structural  purposes  in  the  place 
of  other  sand.  This  dolomite  is  especially  liable,  as  now  shown  by 
extensive  explorations,  to  serious  decay  to  great  depth.  The  under- 
ground circulation  seems  to  attack  the  micaceous  beds  with  great 


NEW  YORK  STATE  MUSEUM 


success  and  in  some  places  the  residue  after  this  solvent  action  is 
of  the  consistency  of  mud.  A  nearly  vertical  attitude  of  the  beds 
accentuates  the  opportunity.  The  most  troublesome  piece  of  ground 
encountered  on  the  whole  line  of  the  New  Croton  aqueduct,  con- 
structed in  1885,  was  in  a  weak  zone  and  crevice  in  the  Inwood  near 
the  village  of  Woodland  on  the  margin  of  the  Sawmill  valley  [see 
discussions  of  Bryn  Mawr  siphon  and  New  York  city  distributions 
in  part  2]. 

The  thickness  probably  varies  but  in  many  places  where  there  is 
only  a  narrow  limestone  belt  it  is  due  more  to  shearing  or  faulting 
out  than  to  original  thinning.  The  most  satisfactory  estimates  are 
based  on  the  explorations  at  Kensico  dam  and  the  field  observations 
at  I52d  street.  They  indicate- an  approximate  thickness  of  700  feet. 
But  in  all  cases  either  the  margins  are  obscured  or  there  is  possibility 
of  faulting  to  modify  measurements.  There  are  no  fossils.  Weath- 
ering and  erosion  has  almost  everywhere  developed  valleys  or  de- 
pressions especially  small  tributary  valleys  in  all  formations,  but  as 
pointed  out  years  ago  by  Professor  Dana  the  principal  valleys  pre- 
vailingly coincide  with  the  limestone  belts. 

(3)  Lowcrre  quartzite.  At  Hastings-on-Hudson  and  again 
near  Croton  lake,  there  is  a  quartzite  that  appears  to  be 
conformable  with  the  Inwood  above.  There  is  possibly  more  than 
50  feet.  It  is  a  simple,  clean  quartzite.  The  other  quartzites  of 
Westchester  and  New  York  county  have  a  more  distinct  relation- 
ship to  the  underlying  gneisses  with  which  they  are  conformable. 
The  Lowerre  of  the  type  locality  is  of  this  second  class.  In  the 
great  majority  of  places  where  this  bed  would  be  expected  to  occur 
there  is  not  a  trace  of  it. 

h  Older  metamorphic  crystallines  (Grenville  series).1  "The 
lowest  and  oldest,  as  well  as  the  most  complex  in  structure  and  rock 
variety,  of  all  the  formations  of  the  Highlands  region  of  south- 
eastern New  York  is  essentially  a  series  of  gneisses."  Cutting 
these  gneisses  as  intrusions  of  various  forms  are  a  great  number 
and  variety  of  more  or  less  distinctly  igneous  types.  In  form  they 
vary  from  small  dikes  or  stringers  to' great  batholithic  masses;  in 
composition,  from  the  extremely  basic  peridotites  or  pyroxinites  of 

1  This  interpretation  of  the  larger  relations  of  the  complex  gneisses 
constituting  the  basis  of  the  series,  lying  below  the  Manhattan-Inwood- 
Lowerre  series,  was  presented  by  the  writer  under  the  title:  Structural 
and  Stratigraphic  Features  of  the  Basal  Gneisses  of  the  Highlands.  N.  Y. 
State  Mus.  Bui.  107  (1907).  p.  361-78.  The  accompanying  description  is 
largely  an  abstract  of  this  paper. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


51 


the  Cortlandt  series  to  the  very  acid  granites  of  Storm  King  moun- 
tain or  the  granophyric  pegmatites  of  North  White  Plains;  and  in 
relative  age  they  likewise  vary  from  a  period  antedating  the  chief 
early  metamorphic  transformation  of  the  Grenville  to  Postman- 
hattan  time.  Put  these  clearly  igneous  types  attain  a  considerable 
prominence  as  separable  units  in  the  practical  consideration  of  the 
problems  of  the  project  and  on  that  account  the  chief  ones  will  be 
more  fully  described  under  the  next  group. 

The  older  portion  —  the  various  schists,  banded  gneisses,  quartz- 
ites,  quartzose  gneisses,  graphitic  schists,  and  serpentinous  and 
tremolitic  limestone,  forming  the  complex  through  which  and  into 
which  the  igneous  masses  have  been  injected  —  form  together  an 
interbedded  series  that  was  originally  a  sedimentary  group.  There 
is  nothing  known  that  is  older  in  this  region.  Us  characteristics  and 
relations  mark  it  as  in  all  probability  the  equivalent  of  the  "  Gren- 
ville "  of  the  Adirondacks  and  Canada. 

No  single  type  and  no  single  characteristic  can  be  given  as  a 
simple  guide  to  the  identification  of  this  formation.  The  prevalence 
of  certain  varieties  or  groups  of  these  and  the  strongly  banded 
structure  give  a  certain  degree  of  character  that  forms  a  reason- 
able working  base.  The  formation  includes  banded  granitic,  horn- 
blendic,  micaceous  and  quartzose  gneisses ;  mica,  hornblende, 
chlorite,  quartz  and  epidote  schists;  garnetiferous,  pyritiferous, 
graphitic,  pyroxenic,  tremolitic,  and  magnetitic  schists  and  gneisses ; 
crystalline,  tremolitic,  and  serpentinous  limestones,  aphi-dolomites, 
serpentines  and  quartzites ;  pyrite,  pyrohitite  and  magnetite  de- 
posits. This  is  the  basal  series.  Put  it  is  complicated  by  a  multi- 
tude of  bands  of  granitic  and  dioritic  gneisses  that  represent 
injections  of  igneous  material  at  a  time  sufficiently  remote  to  be 
subjected  to  most  of  the  early  metamorphic  modifications.  The 
equally  abundant  occurrences  of  quartz  stringers  and  pegmatite 
lenses  though  of  later  origin  can  not  be  separated  from  this  com- 
plex mass  and  the  whole  must  be  regarded  as  a  physical  unit.  The 
occurrence  of  interbedded  limestones  and  quartzites  together  with 
a  variety  of  conformable  schists  and  banded  rocks,  marks  the 
formation  as  essentially  an  old  recrystallized  sediment. 

No  member  of  this  older  unit  of  the  basal  complex  is  sufficiently 
prominent  to  indicate  a  great  break  cr  change  up  to  the  time  of 
the  first  great  dynamic  movements  and  igneous  outbreaks.  The 
following  comparatively  constant  members  are  sometimes  persistent 
enough  to  be  considered  formational  units,  but  even  more  commonly 


52 


NEW  YORK  STATE  MUSEUM 


are  obscure  as  to  boundaries  or  are  of  too  small  development  to 
map  separately. 

(4)  Interbeddcd  quartzite.  Always  a  quartzite  schist  and 
always  exhibiting  conformity  with  the  banded  gneisses  and  schists. 
This  is  regarded  as  the  uppermost  member. 

(5)  Fordham  gneiss  (Banded  gneiss).  Granitic  and  quartzose 
black  and  white  banded  gneisses  and  schists  of  very  complex  com- 
position and  structure. 

(6)  Interbeddcd  limestones.  Crystalline.  Interbedded,  very 
impure,  serpentinous  and  tremolitic.  granular  dolomites,  usually  2 
to  50  feet  thick,  possibly  reaching  a  thickness  of  more  than  100 
feet  in  a  few  cases. 

(7)  Older  intrusive  gneisses.  Variable  types,  mostly  granites  or 
diorites,  strongly  foliated  sills. 

Many  are  of  very  obscure  relations.  The  line  of  close  distinction 
between  recrystallized  sediment,  segregations  accompanying  that 
change,  and  true  igneous  injection  can  not  be  drawn. 

i  Special  additional  igneous  types.  Under  this  heading  are 
included  the  massive  or  little  modified,  not  at  all  or  only  moderately 
foliated,  igneous  masses  of  later  origin  and  local  rather  than  re- 
gional development.  In  some  cases,  however,  they  are  of  decidedly 
controlling  importance  in  the  local  geology  and  rise  to  the  status 
of  definite  formations.  The  most  noteworthy  of  these  within  reach 
of  the  aqueduct  explorations  are : 

(8)  The  Storm  King  Mountain  gneissoid  granite 

(9)  The  Cat  Hill  gneissoid  granite  (central  Highlands) 

(10)  The  Cortlandt  series  of  gabbro-diorites  (near  Peekskill) 

(11)  The  Peekskill  granite  (east  of  Peekskill) 

(12)  The  Ravenswood  granodiorite  (Long  Island  City) 

(13)  The   pegmatite   dikes   and   lenses    (segregational  aqueo- 

igneous  type) 

(8)  The  Storm  King  gneissoid  granite  is  one  of  the  largest  of 
the  clearly  igneous  and  less  completely  foliated  types.  It  consti- 
tutes the  whole  of  Storm  King  mountain  and  the  larger  part  of 
Crows  Nest  on  the  west  side  of  the  Hudson,  and,  crossing  the  river, 
forms  the  chief  rock  of  Bull  hill  and  Breakneck  ridge.  It  is  a 
rather  acid,  coarse  grained,  reddish  granite  with  considerable 
gneissoid  structure  in  a  large  way  [see  Hudson  river  crossings, 
pt2]. 

(9)  The  Cat  Hill  gneissoid  granite  is  not  essentially  different 
from  the  Storm  King  type  as  a  physical  unit.    Its  occurrence  at  a 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


53 


different  point  (Cat  hill),  widely  separated  by  other  types  from 
the  Storm  King  locality,  and  in  rather  large  development,  is  worthy 
of  separate  note.  It  is  cut,  of  course,  in  the  long  tunnel  through 
Cat  hill. 

(10)  The  Cortlandt  series  of  gabbro-diorites  occupies  an  area 
of  about  20  square  miles  between  Peekskill  and  the  Croton  river, 
nearly  all  on  the  east  side  of  the  Hudson.  It  includes  a  very  com- 
plete range  of  coarse  grained,  massive,  igneous  rocks  from  soda 
granites,  grano-diorites  and  quartz-diorites  to  true  diorites,  norites, 
gabbros,  pyroxenites,  and  peridotitcs.  They  doubtless  represent 
stages  or  portions  in  the  differentiation  of  a  magma.  The  inter- 
relations are  only  partially  determinable,  and  the  petrographic  dis- 
tinctions in  detail  are  not  useful  here.  The  area  occupied  by  the 
Cortlandt  series  has  an  uneven  hilly  surface  with  no  structural 
trend,  and  makes  the  most  striking  contrast  to  the  ridge  and  longi- 
tudinal valley  structure  of  the  rest  of  the  region  of  the  crystallines. 

(11)  The  Peekskill  granite,  a  white,  or  pink  massive,  very  coarse 
grained,  soda  granite,  occupying  approximately  4  square  miles  im- 
mediately north  of  the  Cortlandt  area  2  miles  east  of  Peekskill, 
is  believed  to  be  genetically  related  to  the  Cortlandt  series.  The 
evidence  in  favor  of  such  a  relationship  has  been  gathered  in  the 
prosecution  of  this  work  and  has  not  been  published.  But  it  may 
be  said  that  the  textures,  structure,  age,  relationship  to  older  crys- 
tallines, interrelations  with  the  Cortlandt  series,  consanguinity  of 
mineralogy,  and  composition  all  point  toward  the  above  relation- 
ship. In  essential  relations,  therefore,  it  is  the  acid  extreme  of  the 
Cortlandt  series.  Its  economic  features,  however,  are  of  sufficient 
importance  and  its  easy  differentiation  from  the  regular  Cortlandt 
types  require  that  it  should  have  separate  treatment. 

(12)  The  Rravenszvood  grano-diorite  occurs  chiefly  in  Brooklyn. 
It  is  a  slightly  foliated  mass  intrusive  in  the  Fordham  gneiss  and 
is  doubtless  connected  in  origin  with  the  sources  of  many  of  the 
hornblendic  intrusive  bands  in  the  Fordham  and  Manhattan  forma- 
tions in  the  district.  It  covers  a  known  area  of  about  5  or  6  square 
miles  and  may  be  more  extensive.  The  rock  is  suitable  for  struc- 
tural material  and  has  required  consideration  in  the  study  of  "  Dis- 
tributary conduits  "  [see  pt  2  East  River  section]. 

(13)  Pegmatites.  The  pegmatites  and  pegmatitic  granophyric 
masses  of  all  kinds  are  of  almost  universal  distribution  in  the 
foliated  crystallines.  They  vary  from  quartz  bunches  or  stringers 
to  pegmatitic  lenses  and  irregular  masses,  and  to  definite  granitic 


54 


NEW  YORK  STATE  MUSEUM 


or  pegmatic  dikes.  In  many  places  they  constitute  a  large  propor- 
tion of  the  formation  in  which  they  occur.  They  doubtless  vary 
in  age,  but  for  the  most  part  seem  to  belong  to  the  later  period  of 
metamorphism.  Many  of  them  are  massive  and  largely  free  from 
foliation.  They  no  doubt  have  a  complex  origin  between  simple 
aqueous  segregation  on  the  one  side  and  true  igneous  intrusion  on 
the  other. 

Summary  of  formations 

Group  a    Quaternary  deposits 

(i)  Glacial  drift  -v  Occurs    as    a  surface 

Till  and  modified  drift,  extra  mantle  over  nearly  all 
marginal  outwash,  sands  and  >  of  the  region  under 
gravels,  etc.  discussion,  except  the 

immediate  sea  margin 


UNCONFORMITY 

Group  b    Tertiary  and  Cretaceous  deposits 

(2)  Tertiary  outliers 

(a)  Pliocene       littoral  deposits 

(Bridgetons?) 

(b)  Miocene  "  fluffy  "  sand  (Beacon 

hill) 

(3)  Upper  Cretaceous  beds 

(a)  Lignitiferous  sand  (marl  series) 

(b)  Matawan    beds    (clay  marls) 

(c)  Raritan  (clays  and  sands) 


Confined  to  Long  Is- 
land, Staten  Island 
and  the  New  Jersey 

coast 


UNCONFORMITY 

Group  c    Jura-Trias  formations 
(4)  Palisade  diabase  intrusion 


(5)  Newark  series  of  conglomerates, 
sandstones  and  shales 


Confined  to  the  west 
side  of  the  Hudson 
south   of   the  High- 


lands 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


55 


UN  CON  F(  Ik  M  II  V 


Group  d   Dcvonic  strata 

(6)  Catskill,  white  and  red  conglom- 

erate (1725  feet) 

(7)  Oneonta  (upper  flagstone)  (3000 

feet) 

(8)  Ithaca  and  Sherburne  (lower  flag- 

stone) (500  feet) 

(9)  Hamilton    and    Marcellus  shales 

(flagstone    and    shales)  (7°° 
feet) 

(10)  Onondaga  limestone  (200  feet) 

(11)  Esopus     and     Schoharie  shales 

(silicious)  (800  feet) 

(12)  Oriskany  and  Port  Ewen  transi- 

tion (100  feet) 

(13)  Port   Ewen  limestone  and  shale 

(150  feet) 

(14)  Becraft  limestone  (75  feet) 

(15)  New    Scotland    shaly  limestone 

(100  feet) 

(16)  Coeymans   cherty   limestone  (75 
feet) 


Confined  to  the  Cats- 
kills,  the  Esopus  and 
Rondout  valleys,  the 
northern  extension  of 
the  S  h  a  w  a  n  g  u  n  k 
range,  and  Skunne- 
munk  mountain  near 
Cornwall 


Group 


(17)  Manlius  limestone  (70  feet) 

(18)  Cobleskill   limestone   and  cement 

beds  (30  feet) 

(19)  Binnewater  sandstone  (50  feet) 

(20)  High  Falls  shale,  including  small 

limestone  beds  (75-80  feet) 

(21)  Shawangunk  conglomerate  (250- 

350  feet) 


Siluric  strata 

^  Confined  to  the  Rondout 
and  Esopus  valleys 
and  the  northerly  ex- 
tension of  the  Shaw- 
a  n  g  u  n k  range, 
through  the  cement 
region  of  Rosendale, 
Binnewater,  Rondout 
and  Kingston,  and  a 
small  outlier  at  Skun- 
nemunk  mountain 


56 


NEW  YORK  STATE  MUSEUM 


UNCONFORMITY 


Group  f    Cambro-Ordovicic  formations 
(22)  Hudson  River  slates,  shales,  and    Especially  prominent  as 


sandstones    (very  thick)  (Or- 
dovicic)  more  than  2000  feet 
(23)  Wappinger  limestone  (1000  feet) 


( in  part  Cambric  and  in  part  I     valley,  and  the  region 


surface  formations  in 
the  Shawangunk 
range,    the  Wallkill 


Ordovicic) 
(24)  Poughquag  quartzite   (600  feet) 
(Cambric) 


eastward  and  south- 
ward to  the  High- 
lands, on  both  sides 
of  the  Hudson 


Group  g   Later  crystallines  (South  of  the  Highlands) 


(1)  The  Manhattan 


(Uncertain  age 
schist,   a  thor- 


oughly and  coarsely  crystalline 
sediment  of  uncertain  age  — 
generally  supposed  to  be  equiva- 
lent to  the  Hudson  River  slates, 
(Ordovicic)  but  here  separated 
without  necessarily  raising  that 
question  because  of  their  very 
different  physical  and  petro- 
graphic  character 

(2)  Inwood  limestone  (or  dolomite), 

a  magnesian  crystalline  lime- 
stone of  uncertain  age,  generally 
supposed  to  be  the  equivalent  of 
the  Wappinger  (Cambro-Ordo- 
vicic), but  here  enumerated  sep- 
arately without  necessarily  rais- 
ing that  question  because  of 
their  very  different  lithologic 
character  and  associates 

(3)  Lowerre  quartzite,  an  occasional 

quartzite  of  uncertain  relations 
and  very  limited  development 


Confined  to  the  region 
east  of  the  Hudson 
river  and  south  of 
the  Highlands  proper, 
occupying  the  region 
from  the  Highlands 
to  Long  Island 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


57 


UNCONFORMITY 

Group  h    Older  crystallines  (Highlands  gneisses) 

(Grenville  series  of  metamorphics  and  intrusives  —  Precambric) 
(4)  Interbedded  quartzite 


Gren- 
ville 
Series 


A 

quartzose  schist 

(5)  Fordham   gneiss  (chiefly 

sedimentary).  Granitic 
and  quartzose  banded 
gneisses  and  schists  of 
very  complex  develop- 
ment 

(6)  Interbedded 

(Grenville) 
with  the 
gneisses 

(7)  Old  intrusions 

ble  masses  of  granitic  gneisses 
of  igneous  origin  cutting  the 
Grenville  series,  such  as  Storm 
King  granite,  Cat  Hill  granite, 
etc. 


character- 


limestones 
associated 
Fordham 


Large  and  varia- 


High- 
of 


rormations 
istic  of  the 
lands  and  some 
larger  ridges  extend- 
ing southward  to 
New  York  city.  A 
series,  which  in  petro- 
graphic  variety,  is  as 
complex  as  all  of  the 
rest  of  the  forma- 
tions of  the  region 
together 


Postgrenville  in  age 


Group  i    Special  additional 

(8)  Storm    King    gneissoid  granite, 

Storm  King-Breakneck  district 

(9)  Cat  Hill  gneissoid  granite.  Garri- 

son district 

(10)  Cortlandt  series  of  gabbro-diorites. 

Peekskill-Croton  district 

(11)  Peekskill  granite.    A  boss,  related 

to  the  Cortlandt  series.  Peeks- 
kill  district 

(12)  Ravenswood     grano-diorite.  A 

boss.  Brooklyn,  Long  Island 
City  and  Southern  Manhattan 

(13)  Pegmatites.    Dikes,  lenses,  segre- 

gations of  general  distribution 


igneous  types 

~\  These  are  masses  of 
strictly  igneous  origin 
(except  the  pegma- 
tite) and  of  larger 
development  which 
either  because  of 
their  abundance  (peg- 
matites) or  large  area 
(Cortlandt)  or  eco- 
nomic features 
(Peekskill)  or  im- 
portant bearing  upon 
the  plans  of  the  aque- 
duct (Storm  King) 
are  worthy  of  sepa- 
rate note. 


58 


NEW  YORK  STATE  MUSEUM 


3  Major  structural  features 

In  addition  to  the  simpler  structural  characters  of  the  strata, 
already  sufficiently  emphasized  in  the  individual  descriptions,  there 
are  numerous  others  of  more  general  relation  whose  value  and  in- 
fluence it  is  necessary  to  consider  in  many  of  the  practical  problems. 
Those  of  most  importance  are  the  unconformities,  folds  and  faults. 
They  are  directly  related  to  continental  elevation  and  subsidence, 
to  mountain  forming  movements  and  denudation  processes,  to  meta- 
morphism  and  to  igneous  intrusion. 

a  Sedimentation  structures.  In  the  younger  strata  the  prin- 
cipal structures  are  those  of  bedding,  stratification,  conformable 
-succession,  etc.,  characteristic  of  all  sediments  of  such  variety  of 
type.  These  are  prominent  in  the  older  groups  of  formations  down 
to  the  crystallines,  but  the  earlier  Paleozoics  are  also  affected  sc 
profoundly  by  folding  and  faulting  that  attention  is  more  concerned 
with  these  induced  or  secondary  structures. 

Unconformities.  Time  breaks,  with  more  or  less  disturb- 
ance of  strata  and  accompanied  by  erosion,  are  numerous. 

(1)  That  between  the  glacial  drift  and  the  rock  floor  is  the  most 
profound.  It  causes  the  glacial  drift  to  lie  in  contact  with  every 
formation  of  the  region  from  the  oldest  gneisses  of  the  Grenville 
series  of  the  Highlands  to  the  traces  of  Miocene  beds  of  Long 
Island. 

(2)  The  interval  between  the  Pliocene  and  the  Upper  Creta- 
ceous beds  is  more  obscure  and  hardly  reaches  the  importance  of 
an  unconformity.  It  is  probably  more  nearly  of  the  value  of  a 
disconformity  or  of  an  overlap,  and  the  very  limited  development 
of  the  overlying  beds  in  the  region  gives  little  chance  for  determin- 
ing relations  in  much  detail. 

(3)  The  overlap  and  unconformity  between  the  Cretaceous  and 
Triassic.  A  condition  determinable  only  on  the  New  Jersey  side 
of  the  Hudson  river. 

(4)  The  unconformity  between  the  Triassic  and  underlying 
formations  of  different  ages.  An  interval  representing  mountain 
development  and  extensive  erosion*  in  which  the  chief  movement 
probably  belongs  to  the  close  of  Paleozoic  time  and  includes  the 
Appalachian  folding. 

(5)  Unconformity  between  Siluric  and  the  Ordovicic  strata.  An 
interval  representing  mountain  development,  folding  and  erosion, 
in  which  the  movement  known  as  the  Green  Mountain  folding  took 
place. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


59 


(6)  Unconformity  between  the  Poughquag  (Cambric)  quartzite 
and  the  underlying  crystallines.  An  interval  in  all  observable  cases 
of  great  length  and  profound  changes  involving  mountain  folding, 
metamorphism  of  the  profoundest  sort,  and  extensive  erosion. 

(7)  Among  the  crystallines  of  the  south  side  of  the  Highlands 
there  is  one  break  of  similar  importance,  between  the  Inwood  lime- 
stone and  the  underlying  gneisses.  Whether  or  not  it  is  the  same 
as  no.  7  above  is  not  clear,  but  even  if  it  represents  the  same  break 
the  relations  are  somewhat  different  in  degree  and  character  because 
of  the  lack  of  quartzite  in  almost  all  cases. 

Within  the  gneisses  of  the  Grenville  series  and  their  associates 
of  all  kinds  there  are  no  breaks  of  the  unconformity  type  known. 
The  contacts  are  eruptive  in  character,  or  are  displacements 
instead. 

c  Folds  and  mountain-forming  movements.  All  of  the  forma- 
tions from  the  oldest  up  to  and  including  the  Lower  Devonic  strata 
are  folded.  Many  of  the  smaller  (minor)  folds  exhibit  complete 
form  in  the  stream  gorges  of  the  district,  but  all  of  the  larger  ones, 
the  main  folds,  have  in  earlier  time  been  eroded  to  such  extent 
that  the  series  is  beveled  off  and  only  the  truncated  edges  are  to 
be  seen,  exhibiting  strata  standing  more  or  less  perfectly  on  edge, 
and  making  restoration  of  the  form  a  very  difficult  or  impossible 
task.  This  is  only  partially  accomplished  in  the  Siluro-Devonic 
margin  along  the  Shawangunk  range ;  it  is  more  complete  in  the 
Cambro-Ordovicic  north  of  the  Highlands,  and  it  reaches  its  most 
perfect  development  in  the  crystallines  of  the  Highlands  and  New 
York  and  Westchester  counties.  These  differences  correspond 
roughly  to  the  differences  in  age  of  the  strata,  and,  taken  together 
with  the  evidence  of  the  profound  unconformities,  indicate  that 
mountain-forming  movements  of  far-reaching  importance  visited 
the  region  no  less  than  three  times.  Each  time  of  such  disturbance, 
of  course,  the  underlying  older  series  was  affected  by  the  move- 
ments of  that  epoch  in  addition  to  any  previous  ones,  and  as  a  con- 
sequence the  older  is  to  be  expected  to  show  more  complexity  of 
such  structures.  Each  succeeding  series  separated  by  such  activity 
is  therefore  one  degree  simpler  in  structure. 

Of  these  three  epochs  of  great  disturbance,  one  is  (1)  Precambric 
and  corresponds  to  the  time  interval  marked  by  the  unconformity 
between  the  Poughquag  quartzite  and  the  gneisses;  a  second  (2)  is 
Postordovicic  and  corresponds  to  the  time  interval  marked  by  the 
unconformity  between  the  Hudson  River  slates  and  the  Shawan- 


6o 


NEW  YORK  STATE  MUSEUM 


gunk  conglomerates,  and  the  last  (3)  is  Postdevonic  (probably 
Postcarbonic,  judging  from  neighboring  regions  of  similar  history) 
and  has  left  as  its  most  important  evidence  in  this  district,  the 
excessively  complicated  sharp  foldings  and  thrusts  of  the  Shawan- 
gunk  range  and  its  extension  in  the  Rosendale  cement  district. 

Kinds.  As  to  forms  produced  there  are  no  usually  described 
types  that  are  not  to  be  found  here.  The  simpler  forms  of  anti- 
clines and  synclines,  both  open  and  closed,  symmetrical  and  unsym- 
mctrical  and  overturned,  are  all  common.  The  isoclinal  is  common 
in  the  gneisses.  In  each  epoch  of  folding  the  compression  forces 
were  effective  chiefly  in  a  northwest-southeast  direction  producing 
arches  and  troughs  whose  axes  trend  northeast-southwest.  This  i.s 
the  trend  of  the  main  structures  throughout  the  region. 

The  extent  of  crustal  shortening  accomplished  by  this  series  of 
compressions  is  undetermined,  but  that  it  amounts  to  a  total  of 
many  miles  is  indicated  by  the  fact  that  over  broad  areas  the 
strata  stand  almost  on  edge.  Furthermore,  in  the  older  Highlands 
and  in  portions  of  the  Hudson  river  districts  the  folds  have  been 
slightly  overturned  so  that  commonly  the  strata  on  both  limbs  dip 
in  the  same  direction  (toward  the  southeast).  This  seems  to  indi- 
cate a  strong  thrust  from  the  southeast.  All  stages  between  the 
gentlest  warping  to  strongly  overturned  folds,  and  from  minute 
crumbling  to  folds  of  great  extent  and  persistence  are  to  be  seen. 

The  effect  of  all  the  folding  is  chiefly  to  present  a  series  of  up- 
turned strata  to  erosion  and  encourage  a  subsequent  development 
of  valleys  along  the  softer  beds  bordered  by  ridges  of  the  more 
resistant  types. 

As  the  axes  of  the  folds  lie  in  a  northeast-southwest  direction, 
this  gives  a  marked  physiographic  development  of  ridges  and  val- 
leys of  the  same  trend,  a  most  conspicuous  topographic  feature  of 
southeastern  New  York. 

d  Faults.  Accompanying  the  folding  in  each  epoch,  and 
especially  the  stronger  overthrust  movements  there  has  been  a 
tendency  to  rupture  and  displacement.  These  breaks  are  known 
as  faults.  Multitudes  of  them  are  of  minute  proportions  and  prac- 
tically neglectable  in  a  broad  view,  but  many  also  are  of  large 
extent,  traceable  across  country  for  many  miles  and  indicating  dis- 
placements in  some  cases  of  many  hundreds  of  feet.  For  the  most 
part  these  faults  are  of  the  thrust  type  and  wholly  consistent  with 
the  folds  in  origin.  They  run  generally  in  a  northeast-southwest 
direction,  especially  the  larger  ones,  and  frequently  form  the  sep- 
aration planes  between  different  formations.     Occasional  cross 


3 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  6l 

faults  occur  (with  northwest-southeast  direction  across  the  strike), 
but  so  far  as  is  known  they  are  always  of  minor  consequence.  In 
rare  instances,  the  trace  of  a  fault  line  on  the  surface  describes 
curious  curves,  such  as  that  at  Cronomer  hill  above  Newburgh, 
apparently  inconsistent  with  the  chief  structural  trend,  but  a  study 
of  the  whole  geologic  relation  in  such  cases  shows  them  to  be  con- 
nected with  the  projecting  spurs  of  underlying  formations  which 
in  any  large  thrust  movement  plow  their  way  with  some  success 
through  the  younger  overlying,  less  resistant,  strata.  They  differ 
in  no  material  way  from  the  ether  more  simple  looking  lines. 

Both  normal  and  thrust  faults  occur,  but  the  thrust  type  appears 
to  be  most  common. 

The  amount  of  displacement  or  throw  is  extremely  variable.  The 
larger  faults  represent  movements  of  several  hundred  feet.  In 
rare  cases  the  movement  may  be  as  much  as  2000  feet. 

The  effects  may  be  grouped  as  follows:  (1)  the  appearance  of 
formations  out  of  their  normal  order,  i.  e.  contacts  between  forma- 
tions that  do  not  normally  lie  next  to  each  other;  (2)  the  produc- 
tion of  escarpments,  i.  e.  steep  cliff-bordered  ridges;  (3)  the  de- 
velopment of  zones  of  more  or  less  extensively  crushed  rock  along 
the  principal  plane  of  movement;  (4)  the  determination  of  loca- 
tion for  stream  courses  and  gulches  and  valleys  that  cross  the 
formations. 

All  of  these  effects  are  more  noticeable  and  better  preserved  for 
the  later  movements  than  for  the  earlier  ones.  Many  of  those 
dating  back  to  the  earliest  epoch,  affecting  only  the  crystalline  rocks 
of  the  Highlands,  are  not  readily  detected.  Most  of  the  breaks 
have  been  healed  by  recrystallization  and  the  contacts  are  often 
as  close  and  sound  as  any  other  part  of  the  formation. 

But  this  is  not  so  true  of  the  later  epochs  —  and  in  them  a  good 
deal  depends  upon  the  type  of  rock  affected.  The  more  brittle  and 
hard  and  insoluble  types  are  more  likely  to  still  have  open  seams 
and  unhealed  fractures  than  the  softer  and  more  easily  molded 
formations.  In  some  of  these,  recent  water  circulation  has  still 
further  injured  the  fault  zones  by  introducing  rock  decay  to  con- 
siderable depth.  Because  of  the  more  ready  circulation  in  them,  it 
is  noticeable  that  some  of  the  extensive  decay  effects  are  produced 
in  crystalline  rocks  that  otherwise  very  successfully  resist  destruc- 
tion. On  the  whole  the  softer  clay  shales  and  slates  are  less  likely 
to  preserve  open  water  channels  of  this  sort  than  any  other  forma- 
tion of  the  region. 


62 


NEW  YORK  STATE  MUSEUM 


No  part  of  the  region  is  wholly  free  from  faulting  effects,  except 
perhaps  a  part  of  Long  Island.  The  Catskills  also  are  very  little 
affected  —  so  little  that  this  type  of  structure  has  not  require  con- 
sideration in  the  vicinity  of  Ashokan  reservoir.  But  all  parts  of 
both  the  northern  and  southern  aqueduct  system  have  had  this 
feature  to  consider. 

Further  discussion  of  the  specific  local  prohlems  introduced  by 
faulting  and  folding  is  given  under  the  problems  of  part  2.  A  con- 
siderably more  extended  comment  on  the  age  of  fault  movement 
is  given  under  the  heading  "  Postglacial  faulting." 

4  Outline  of  geologic  history 

Most  of  the  genera!  features  of  geologic  history  have  been 
involved  more  or  less  in  the  foregoing  discussion.  It  is  im- 
possible to  wholly  separate  matters  that  are  so  intimately  inter- 
related even  though  it  is  convenient  to  think  of  or  consider  one 
phase  at  a  time.  But  it  may  serve  a  useful  purpose  to  summarize 
the  steps  of  progress  as  illustrated  by  local  geology  from  the  earliest 
geologic  time  to  the  present. 

a  Earliest  time.  (Prepaleozoic,  Agnotozoic,  Proterozoic,  or 
Azoic  Era).  There  is  little  doubt  that  the  oldest  rocks  known  in 
this  region  are  representatives  of  a  time  of  regular  sedimentation. 
Conditions  favored  the  deposition  of  silicious  detritus  of  variable 
composition  with  an  occasional  deposition  of  lime,  nearly  always  in 
very  thin  beds.  What  these  sediments  were  laid  down  upon  or 
where  they  came  from  are  unsolved  questions.  The  remnants  of 
them  that  are  still  preserved  are  the  basis  of  the  "  Grenville  series  " 
as  interpreted  in  this  area,  and  are  the  basal  (oldest)  members  of 
the  "  Fordham  "  or  "  Highlands  gneisses." 

How  long  ago  this  series  was  deposited  is  not  known.  It  can  be 
stated  only  approximately  even  in  the  rather  flexible  terms  used  in 
historical  geology.  It  is  older  than  any  Paleozoic  strata  (Pre- 
cambric),  probably  very  much  older.  It  is  even  possible  that  this 
series  is  as  much  older  than  the  Cambric  as  that  period  is  compared 
to  the  present.  In  short,  it  is  not  known,  and  there  is  apparently 
little  immediate  likelihood  of  finding  out  even  to  which  of  the  sev- 
eral subdivisions  of  the  Prepaleozoic  this  series  belongs.  It  is  cer- 
tain that  before  the  Cambric  sandstones  of  the  Paleozoic  era  had 
begun  to  form,  this  older  series  was  disturbed  by  crustal  move- 
ments, folded,  metamorphosed,  intruded  by  igneous  injections,  ele- 
vated above  the  water  (sea)  level  of  that  time  and  eroded  by  sur- 
face agencies.    These  movements  and  steps  there  is  no  doubt  of. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


63 


When  subsidence1  again  depressed  the  area  beneath  the  sea  the 
deposition  of  sands  that  we  now  call  Cambric  (Poughquag)  quartz- 
ite  began. 

/;  Early  Paleozoic  time.  With  the  sedimentation  upon  this  old 
crystalline  rock  floor  a  long  time  of  apparently  continuous  deposi- 
tion began  which  ultimately  resulted  in  the  accumulation  of  several 
thousand  feet  of  sandstones,  limestones,  and  sandy  or  clayey  shales 
that  are  now  known  as  the  Cambro-Ordovicic  series  (Poughquag- 
Wappinger-Hudson  River  series).  But  at  the  close  of  Ordovicic 
time  or  late  in  that  period  another  crustal  revolution  began.  The 
whole  region  was  again  compressed  into  mountain  folds,  faulted, 
sheared,  metamorphosed,  elevated  above  sea  level,  and  subjected  to 
erosion.  This  corresponds  to  the  Green  mountains  folding  of 
Vermont. 

With  the  next  subsidence  and  a  return  of  sedimentation  a  new 
series  began  to  form.  The  break  marking  the  occurrence  of  all 
these  changes,  known  locally  as  the  Postordovicic  unconformity, 
represents  a  considerable  portion  of  Siluric  time. 

c  Middle  Paleozoic  time.  The  earliest  deposits  of  this  series, 
which  continued  to  accumulate  through  late  Siluric  and  all  of  De- 
vonic  time,  were  heavy  conglomerates  very  unevenly  distributed 
over  the  new  rock  floor.  These  are  the  so  called  Shawangunk  con- 
glomerates, a  formation  that  within  the  boundaries  of  this  imme- 
diate area  and  within  a  distance  of  20  miles  varies  from  a  thickness 
of  more  than  300  feet  to  almost  nothing.  But  for  the  most  part, 
sedimentation  was  regular  and  fairly  continuous  and  of  immense 
volume.  The  whole  series  of  conglomerates,  sandstones,  shales, 
grits  and  limestones  belonging  to  the  later  Siluric  and  the  Devonic 
are  included.  Not  all  are  believed  to  be  marine  however.  The 
Catskill  and  Shawangunk  conglomerates  may  well  be  of  continental 
type. 

Long  after  the  deposition  of  all  of  these  strata  another  crustal 
disturbance,  for  at  least  the  third  time,  repeated  the  process  of 
mountain-folding  and  erosion.  This  was  the  time  of  the  Appalach- 
ian mountain-folding.  In  this  region  it  caused  a  wonderfully  com- 
plex development  of  folds  and  faults  that  are  especially  important 
and  determinable  as  to  type  and  age  in  the  Rondout  cement  region. 
The  movement,  of  course,  affected  all  of  the  older  formations  as 

1  There  may  possibly  be  an  intermediate  stage,  practically  a  duplication 
of  the  whole  as  given  above,  between  the  very  oldest  and  the  Cambric, 
represented  in  the  "  later  crystallines,"  but  this  may  as  well  be  neglected 
for  the  present. 


64 


NEW  YORK  STATE  MUSEUM 


well,  but  on  them,  already  disturbed  by  earlier  displacements,  the 
features  chargeable  to  the  disturbance  can  not  always  be  distin- 
guished from  older  ones.  All  three  of  the  mountain-forming  com- 
pressions seem  to  have  been  controlled  by  the  same  relationship 
of  forces  and  adjustments  of  movement,  for  the  results  are  in  each 
case  the  production  of  folds  or  faults  of  similar  orientation  and  a 
final  structure  of  uniform  trend. 

Deposition  had  been  going  on  for  ages,  chiefly  on  the  west  and 
north  side  of  the  older  crystallines ;  but  with  a  return  of  sedimenta- 
tion a  decided  reversal  is  noted.  The  Atlantic  border  is  depressed 
and  much  of  the  interior  region  seems  not  to  have  been  subjected 
to  further  deposition  from  that  time  even  to  the  present. 

d  Mesozoic  time.  Again  conglomerates,  sandstones  and 
shales  were  laid  down  upon  an  eroded  floor.  From  their  condition 
and  lithology  it  is  believed  that  they  are  partly  of  continental,  flood 
plain,  origin.  The  series  is  thick,  generally  assigned  to  the  Triassic 
period  and  is  extensively  developed.  During  the  time  of  accumula- 
tion and  to  some  extent  subsequent  to  it,  there  was  extensive 
igneous  activity  pouring  out  and  intruding  basic  basaltic  matter  in 
large  amount.  The  Palisade  diabase  sill,  and  the  Watchung  Moun- 
tain basalt  flows  are  the  best  examples. 

At  a  later  time  small  faulting  occurred  making  frequent  dis- 
placements in  this  series.  But  mountain-folding  has  not  again 
visited  the  region.  Such  breaks  as  there  are,  are  of  the  nature  of 
overlaps  and  disconformities  rather  than  of  the  revolutionary  his- 
tory indicated  by  a  true  unconformity.  One  of  these  intervals 
occurs  in  the  Mesozoic  between  the  Triassic  and  Cretaceous.  Above 
it  the  thick  series  of  Cretaceous  shales,  marls,  sands  and  clays  are 
developed.  Succeeding  this  series  a  similar  interval  represents  the 
earliest  Cenozoic  time. 

c  Early  Cenozoic  time.  The  earliest  Cenozoic  (Eocene  and 
Oligocene)  has  no  sedimentary  record  within  this  region. 

There  are  small  remnants  of  deposition  representing  Miocene  and 
Pliocene  time.  Above  these  again  the  record  is  blank  up  to  the 
time  of  the  glacial  invasion. 

/  Late  Cenozoic  time  —  glacial  period.  By  some  combina- 
tion of  conditions  not  very  well  understood,  the  chief  features  of 
which  no  doubt  are, —  (i)  continental  elevation  and  (2)  shifting 
of  centers  of  precipitation  and  (3)  modification  in  the  composition 
of  the  atmosphere,  a  period  of  excessive  ice  accumulation  was 
inaugurated.    Ice  finally  covered  immense  continental  areas  and 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


65 


from  its  own  weight  by  continuous  accumulation  spread  out 
(flowed)  from  great  central  areas  toward  the  margins.  There  is 
clear  evidence  of  interruptions  or  advances  and  retreats  of  this 
general  movement  many  times.  But  the  same  type  of  work  and 
similar  results  were  attained  in  each  case.  The  chief  features  of 
this  work  was  the  moving  of  rock  material  frozen  in  the  ice  to  long 
distances  and  the  deposition  of  it  again,  more  or  less  modified  by 
its  contact  with  the  ice  or  by  the  effect  of  water  upon  its  release, 
at  other  places  and  with  entirely  new  associations.  The  tendency 
to  ice  accumulation  was  finally  overcome  to  sufficient  extent  for  the 
inauguration  of  the  present  condition  of  things.  Whether  it  is  a 
permanent  change  or  only  an  interglacial  interval  is  not  clear. 
But  the  ice  has  withdrawn  to  the  mountains  and  the  polar  north 
at  the  present  time.  It  has  not  occupied  the  surface  of  this  region 
probably  within  the  last  40,000  years,  and  perhaps  for  a  much 
longer  time. 

5  Outline  of  geographic  history  —  physiography 

The  surface  features  of  a  country  are  the  result  of  the  working 
out  of  a  long  and  complex  series  of  processes  with  and  upon  the 
materials  of  the  rock  floor  or  bed  rock.  The  relationship  of  surface 
features  to  the  formations  that  occur  in  the  rock  floor  and  their 
stages  of  development,  in  short,  an  interpretation  of  their  origin  and 
meaning,  constitutes  geographic  history  or  physiography.  It  differs 
little  in  essential  character  from  geologic  history,  of  which  it  is  only 
a  special  branch,  i.  e.  the  history  of  surface  configuration.  And  it 
can  not  be  appreciated  or  understood  except  in  the  light  of  a 
thorough  knowledge  of  stratigraphic  and  structural  geology.  In 
individual  cases  or  particular  regions  the  geologic  knowledge  must 
also  be  specific. 

a  Early  stages.  Occasional  glimpses  of  surface  features,  and 
some  scattered  facts  about  their  development  are  to  be  gathered  of 
older  continental  existence.  Surface  features  characteristic  of  their 
time  were  developed  in  the  great  intervals  between  each  successive 
period  of  continuous  deposition.  Traces  of  them  are  involved  in  the 
unconformities  of  the  geologic  column  already  shown  in  the  discus- 
sion of  geologic  history.  Hills,  valleys,  streams,  shores  and  all  the 
appropriate  assortment  of  forms  must  have  existed.  But  they 
could  not  have  been  like  those  of  the  present  in  many  minor  fea- 
tures —  especially  in  arrangement  and  distribution  —  because  the 
bed  rock  of  those  times  had  only  in  part  reached  the  complexity  of 


66 


NEW  YORK  STATE  MUSEUM 


structure  and  composition  now  belonging  to  it.  Many  items  of  im- 
portance are  indicated  in  some  of  these  early  periods.  For  ex- 
ample, the  sea  encroached  on  the  land  borders  repeatedly  from  the 
westward  —  especially  throughout  Palezoic  times,  while  in  Meso- 
zoic  and  Cenozoic  times  the  evidence  of  shif tings  of  sea  margins 
is  confined  to  the  east  and  southeast  borders,  and  likewise  probably 
no  near  by  place  has  been  continuously  beneath  the  sea. 

But  the  unraveling  of  these  conditions  is  obscured  by  subsequent 
events.  Land  surfaces  that  once  were,  became  covered  by  later 
sediments.  The  physiography  of  those  times,  Paleophysiography,  as 
well  as  paleogeography,  is  therefore  a  difficult  and  intricate  line  of 
investigation.  With  these  ancient*  surfaces  the  dicussion  of  present 
features  has  little  to  do.  Here  and  there  the  present  surface  cuts 
across  and  exposes  the  edges  of  an  older  one  giving  traces  of  the 
old  profile ;  but  in  most  cases  it  is  so  distorted  by  the  foldings  and 
other  displacements  belonging  to  a  later  period  that  a  restoration 
of  the  original  continental  features  is  a  task  fit  for  the  most  highly 
trained  specialist. 

The  surface  as  it  now  exists,  and  the  rock  floor  modified  only  by 
the  inequalities  of  the  loose  soil  mantle,  yields  more  readily  to  in- 
vestigations of  origin  and  history. 

b  History  of  present  surface  configuration.  On  some  por- 
tions of  the  region  there  seems  to  have  been  no  deposition  since  the 
close  of  Paleozoic  time.  Throughout  most  of  Mesozoic  and  Ceno- 
zoic times,  therefore,  those  regions  probably  have  been  continuously 
land  areas  (continental)  and  have  been  subjected  to  the  agencies 
of  erosion.  This  applies  particularly  to  the  Highlands  region  and 
the  Catskills  and  the  Shawangunk  range  and  intervening  country. 

What  the  surface  configuration  was  like  in  the  early  stages  is 
wholly  unknown.  In  the  beginning,  mountain-folding  —  the  Appa- 
lachian folding  —  was  in  progress  and  the  features  were  probably 
those  of  partially  dissected  anticlinal  folds.  With  the  progress  of 
erosion  the  Triassic  deposits  were  accumulated  along  the  eastern 
border,  probably  on  the  continental  slopes.  Subsequently,  further 
elevation  extended  erosion  over  the  Triassic  areas  also  and  the 
Cretaceous  beds  were  laid  down  on  the  margin.  The  general  lines 
of  development  have  been  the  same  from  that  time  to  the  present. 
Each  successive  important  formation  less  heavily  developed  and 
forming  a  band  outside  of  and  upon  the  older  one  —  the  whole  now 
constituting  a  series  of  successive  belts  the  oldest  of  which  is  far 
inland  and  the  newest  at  the  sea  margin. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


67 


Therefore,  when  long  periods  of  denudation  are  referred  to,  it  is 
well  to  appreciate  that  this  is  especially  applicable  to  the  interior, 
that  the  sea  margins  are  comparatively  new,  and  that  certain  of  the 
inland  areas  were  suffering  erosion  long  before  the  rock  forma- 
tions that  lie  beneath  and  form  the  rock  floor  of  the  sea  border 
districts  were  in  existence. 

Cretaceous  peneplain.  It  appears  from  studies  of  these  problems 
in  a  broad  way,  and,  drawing  upon  generalizations  from  continental 
features  of  a  much  larger  field  than  that  of  the  present  study,  that 
the  continental  region  of  which  this  forms  a  part  must,  in  the 
earlier  periods,  have  remained  in  comparatively  stable  equilibrium 
for  an  extraordinarily  long  time.  So  long  a  time  elapsed  that  most 
of  the  area  was  reduced  by  erosion  to  a  monotonous  plain  (pene- 
plain) at  a  very  low  altitude,  probably  not  much  above  the  sea 
(base  level).  Only  here  and  there  were  there  areas  resistant  enough 
or  remote  enough  to  withstand  the  denuding  forces  and  stand  out 
upon  the  general  plain  as  remnants  of  mountain  groups  (Monad- 
nocks).  Possibly  the  Catskill  mountains  of  that  day  had  such 
relation. 

This  reduction  of  surface  feature  it  is  believed  was  reached  in 
late  Cretaceous  time.  The  continent  stood  much  lower  than  now. 
Portions  that  are  now  mountain  tops  and  the  crests  of  ridges  were 
then  constituent  parts  of  the  rock  door  of  the  peneplain  not  much 
above  sea  level.  This  rock  floor  was  probably  thickly  covered  with 
alluvial  deposits  (Hood  plain)  not  very  different  in  character  from 
the  alluvial  matter  of  portions  of  the  lower  Mississippi  valley  of 
today. 

Upon  such  a  surface  the  principal  rivers  of  that  time  flowed, 
sluggishly  meandering  over  alluvial  sands  and  taking  their  courses 
toward  the  sea  (the  Atlantic)  in  large  part  free  from  influence  by 
the  underlying  rock  structure.  The  ridges  and  valleys,  the  hills, 
mountains  and  gorges  of  the  present  were  not  in  existence,  except 
potentially  in  the  hidden  differences  of  hardness  or  rock  structure. 
Such  conditions  prevailed  over  a  very  large  region  —  certainly  all 
of  the  eastern  portion  of  the  United  States.  This  so  called  Creta- 
ceous peneplain  is  the  starting  point  in  development  of  the  geo- 
graphic features  of  the  present. 

Continental  elevation.  Following  upon  this  period  of  stability 
and  extensive  denudation  came  one  of  continental  elevation.  How 
much  above  sea  level  this  raise:!  the  areas  under  present  discussion 
may  not  be  determined,  but  that  it  was  a  sufficient  amount  to 


68 


NEW   YORK  STATE  MUSEUM 


rejuvenate  the  streams  and  permit  them  to  begin  the  sculpturing 
of  the  land  in  a  new  cycle  of  erosion  is  perfectly  clear.  As  soon 
as  the  elevation  and  warping  of  the  continental  border  made  its 
influence  felt  in  the  increased  activity  and  efficiency  of  the  streams 
(rejuvenation)  they  began  transporting  the  alluvium  of  their  flood 
plains  and  to  sink  their  courses  through  this  loose  material  to  bed 
rock.  The  final  result  of  long  continued  denudation  under  these 
conditions  in  early  Tertiary  time  was  the  removal  of  the  loose 
mantle  and  the  beginning  of  attack  on  bed  rock  (superimposed 
drainage).  The  streams  formerly  flowing  on  alluvium  that  had 
now  cut  down  to  rock  found  themselves  superimposed  upon  a 
rock  structure  not  at  all  consistent  with  their  former  courses. 
With  the  progress  of  erosion  on  this  rock  floor  all  these  differ- 
ences of  structure,  such  as  the  differences  in  hardness  of  beds, 
the  trend  of  the  folds,  the  strike  of  the  faults,  the  igneous  masses, 
etc.,  were  discovered  and  the  streams  began  to  adjust  their  courses 
to  them.  Valleys  were  carved  out  where  belts  of  softer  rock 
occur,  ridges  were  left  as  residuary  remnants  where  belts  of  harder 
rock  exist,  and  the  surface  (relief)  took  on  some  of  the  char- 
acter of  present  day  lines.  That  is,  the  principal  mountain  ranges 
of  that  time  were  the  same  as  those  of  today  in  position  and 
trend ;  but  they  had  not  so  great  apparent  hight  because  the  in- 
tervening valleys  had  not  yet  been  cut  so  deep.  The  principal 
escarpments  of  that  time  were  due  to  the  same  structural  lines 
as  those  of  today,  only  they  have  shifted  somewhat  along  with 
the  general  retreat  of  all  prominences  by  the  forces  of  weathering 
and  erosion. 

In  the  course  of  this  work  of  sculpturing  and  the  shifting  of 
valleys  and  divides  and  escarpments  and  barriers  into  constantly 
greater  and  greater  conformity  with  rock  structure,  it  came  about 
by  and  by  that  practically  all  of  the  smaller  and  tributary  streams 
had  so  completely  adjusted  themselves  to  their  geologic  environ- 
ment that  their  valleys  almost  everywhere  followed  along  the 
softer  beds  (subsequent  streams),  the  divides  were  chiefly  of 
harder  beds,  the  trend  of  both  were  almost  everywhere  parallel  to 
the  strike  of  the  rock  folds  and  other  structures  (adjusted  drainage) 
This  undoubtedly  involved  in  many  cases  a  very  radical  change  of 
stream  course,  and  in  some  cases  an  ultimate  reversal  of  drainage 
to  such  extent  that  tributaries  were  deflected  inland  against  the 
course  of  the  master  streams  and  in  some  cases  actually  flowed 
many  miles  in  this  reversed  direction  before  finding  an  accordant 
junction  (retrograde  streams).    At  least  three  of  the  streams  of 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


69 


southeastern  New  York  are  still  of  this  type  —  the  Wallkill,  the 
Rondout  and  the  lower  portion  of  the  Esopus. 

But  the  larger  rivers,  the  great  master  streams,  of  the  super- 
imposed drainage  system,  in  some  cases  were  so  efficient  in  the 
corrasion  of  their  channels  that  the  discovery  of  discordant  struc- 
tures has  not  heen  of  sufficient  inlluence  to  displace  them,  or  re- 
verse them,  or  even  to  shift  them  very  far  from  their  original  direct 
course  to  the  sea.  They  cut  directly  across  mountain  ridges  be- 
cause they  flowed  over  the  plain  out  of  which  these  ridges  have 
been  carved  and  because  their  own  erosive  and  transporting  power 
have  exceeded  those  of  any  of  their  tributaries  or  their  neighbors. 
They  are  superimposed  streams  (not  antecedent),  they  have,  with 
their  tributaries,  settled  down  in  the  ancient  plain,  and,  by  their 
own  erosive  activity,  have  carved  the  valleys  deeper  and  deeper, 
cutting  the  upland  divides  narrower  and  narrower  until  now  only 
here  and  there  a  ridge  or  a  mountain  remnant  stands  with  its  crest 
or  summit  almost  reaching  up  to  the  level  of  the  ancient  pene- 
plain on  which  the  work  began,  if  the  transported  matter  could 
all  be  brought  back  and  replaced  in  these  valleys  the  old  plain 
might  be  restored,  but  the  work  would  immediately  begin  al!  over 
again. 

Of  these  great  master  streams  the  Hudson  is  the  only  local  rep- 
resentative [see  Study  of  the  Hudson  River  gorge  in  part  2]. 

Tertiary  incomplete  pencplanation.  Such  processes,  if  allowed  to 
continue  on  a  stable  continental  region,  would  ultimately  reduce 
the  land  for  a  second  time  to  a  monotonous  plain  (complete  cycle 
of  erosion).  The  beginnings  of  such  a  plain  would  be  made  in 
the  principal  stream  valleys  upon  reaching  graded  condition.  Their 
lateral  planation  and  the  development  of  flat-bottomed  valleys 
would  begin  at  about  the  level  that  the  plain  would  stand  in  the 
final  completed  stage.  The  difference  of  elevation  between  the 
ridge  crests  or  hilltops  and  these  flat  valleys,  i.  e.  between  the  old 
peneplain  and  the  new  unfinished  one  would  be  an  approximate 
measure  of  the  amount  of  the  continental  elevation  that  instituted 
the  new  cycle. 

But  judging  from  such  remnants  of  this  later  plain  as  are  to 
be  seen,  the  two,  i.  e.  the  old  Cretaceous  peneplain  and  the  new 
Tertiary  peneplain  are  not  parallel.  Toward  the  southeast,  toward 
the  sea,  the  older  plain  descends  more  rapidly  than  the  younger 
and  intersects  it.  Both  pass  beneath  sea  level  in  that  direction. 
The  difference  between  them  therefore  varies  with  locality  from 


7° 


NEW   YORK  STATE  MUSEUM 


o  feet  to  perhaps  2000  feet  within  the  borders  of  the  area  (con- 
tinental tilting  or  warping). 

Late  Tertiary  rcclevation.  Traces  of  such  an  intermediate  and 
incomplete  peneplain  are  to  be  seen  in  the  compound  nature  of  the 
large  valleys  of  the  present  day.  Most  of  them  are  essentially  broad 
valleys  into  the  bottoms  of  which  narrower  valleys  and  gorges  are 
cut.  The  tops  of  the  minor  hills  and  ridges  of  the  broad  valleys 
represent  the  intermediate  Tertiary  peneplain  that  was  interrupted 
in  its  development  before  completion  (interrupted  erosion  cycle). 
The  inner  narrow  valleys  indicate  that  for  the  second  time  a  re- 
gional elevation  rejuvenated  the  streams  and  they  began  their 
work  of  cutting  to  a  new  grade.  They  have  made  a  good  begin- 
ning at  this  task,  and  as  a  consequence  have  carved  some  rebel 
in  the  old  valley  bottoms.  These  new  streams  have  not  yet  reached 
a  graded  condition. 

When  the  glacial  ice  began  to  invade  this  region  all  of  the  surface 
features  had  had  such  a  history.  Leaving  out  of  account  minor 
fluctuations  of  elevation  and  depression,  of  which  there  may  have 
been  several  of  too  transient  character  to  make  a  lasting  impres- 
sion on  the  topography,  the  stages  become  comparatively  few  and 
the  general  tendencies  are  easily  understood. 

The  measurable  differences  of  elevation  between  the  Cretaceous 
and  Tertiary  peneplains  give  some  reasonable  conception  of  the 
amount  of  the  first  continental  or  regional  elevation.  Concerning 
the  altitude  reached  in  subsequent  regional  elevation  there  is  less 
certainty.  None  of  the  streams,  not  even  the  master  streams  such 
as  the  Hudson,  reached  grade,  for  it  exhibits  strictly  a  gorge  type 
not  only  within  the  present  land  borders,  but  it  is  now  known  to 
show  gorge  development  far  beyond  the  present  coast  line.  Judg- 
ing from  the  Hudson,  therefore,  it  seems  necessary  to  conclude 
that  this  continental  region  stood  at  a  much  greater  elevation  in 
some  portions  of  the  later  period  than  had  formerly  prevailed. 
Probably  the  maximum  elevation  immediately  preceded  the  glacial 
invasion. 

Conservative  estimates  as  to  the  amount  of  elevation  of  that 
time  in  excess  of  the  present  would  place  it  at  not  less  than  2000 
feet.  Much  more  than  that  is  believed  to  be  indicated,  possibly 
5000  feet  or  more. 

In  the  meantime,  the  master  stream,  the  Hudson  and  several 
of  the  tributaries  cut  into  their  valley  bottoms  to  such  extent  as 
to  make  typical  gorges  so  deep  that  their  beds  now,  since  the  sub- 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


7' 


sidence,  lie  much  below  sea  level.  The  Hudson  bed  is  of  this 
character  throughout  its  course  from  Albany  to  the  Atlantic,  and 
in  the  Highlands,  60  miles  inland,  the  known  rock  bed  at  one  point 
is  more  than  700  feet  below  sea  level. 

In  late  glacial  time  there  was  still  greater  subsidence  (50-100 
feet)  than  the  present  as  is  indicated  by  terraces  above  present 
water  level  and  the  deltas  formed  at  the  mouths  of  tributary 
streams. 

Such  in  general  outline  is  the  history  of  successive  conditions 
governing  the  topographic  development  of  the  rock  floor.  The  suc- 
cession of  periods  of  stability,  elevation,  stability  again,  reelevation 
and  subsidence  have  had  an  effect  on  all  sorts  of  formations,  but 
the  extent  of  the  impress  and  its  permanence  varies  greatly  in 
the  different  districts.  Tt  is  not  possible  to  study  these  differences 
in  detail  here.  They  are  the  minor  and  special  local  characters  that 
are  in  control  at  particular  localities.  In  discussions  of  special 
problems  some  of  these  are  taken  up  in  more  detail.  But  in  each 
case  the  general  history  as  outlined  above,  together  with  the  modi- 
fying influence  of  known  local  structure  and  stratigraphic  char- 
acter are  the  foundations  of  a  working  understanding  [see  Hudson 
River  crossings,  Moodna  creek,  Rondout  valley,  etc.,  pt  2]. 

Pleistocene  glaciation.  An  additional  modification  and  one  largely 
independent  of  and  largely  inconsistent  with  the  distribution  of  the 
smaller  features  of  the  rock  floor  is  introduced  by  the  glacial  drift. 
It  covers  almost  everything,  but  so  unevenly  as  to  largely  destroy 
some  of  the  detail.  It  is  in  places  more  than  350  feet  thick  (as 
in  the  Moodna  and  Rondout  valleys')  and  in  others  it  amounts  to 
nothing.  Tt  covers  the  narrow  ravines  and  gorges  heaviest  and 
has  altered  the  courses  of  many  of  the  smaller  streams,  the  original 
channels  being  hopelessly  buried.  The  result  has  been  chiefly  one 
of  reducing  the  ruggedness  of  outline  that  prevailed  along  the 
newer  gorges  of  late  preglacial  time. 

Besides  this  the  usual  surface  forms  characteristic  of  glacial  de- 
posits, occur — -the  kame,  the  drumlin,  the  esker,  the  hill  and  ket- 
tle topography  of  the  terminal  moraine,  the  overwash  plain,  the 
delta,  the  lake  deposit  and  the  gentle  undulations  of  the  ground 
moraine.  These  are  superimposed  on  the  rock  floor  features.  Both 
are  equally  important  to  understand  in  the  problems  that  have  been 
encountered.  Which  set  of  factors  is  to  be  most  regarded  in  a 
given  case  depends  wholly  upon  the  locality  and  the  kind  of  en- 
terprise or  work  it  is  proposed  to  undertake. 


72 


NEW  YORK  STATE  MUSEUM 


c  Physiographic  interpretation.  Rock  floor  contour  is  an  ex- 
pression of  the  differences  in  character  and  structure  of  the  bed 
rock  formations  themselves,  brought  about  by  ordinary  surface 
weathering  and  transporting  agencies,  varied  in  their  action  and 
effects  only  by  certain  differences  in  elevation  above  the  sea.  It 
is  apparent  therefore  that  it  would  be  possible  by  careful  observa- 
tion of  surface  features  to  gather  data  sufficiently  definite  to  fur- 
nish a  basis  for  suggestions  about  hidden  and  hitherto  unknown 
or  undiscovered  structural  and  stratigraphic  characters.  But  the 
application  of  it  to  practical  engineering  problems  is  a  complicated 
and  difficult  matter.  And  this  difficulty  is  nowise  simplified  by 
the  occurrence  of  a  drift  soil  that  tends  to  obscure  many  of  the 
more  delicate  features.  For  example,  the  later  narrow  stream 
gorges  marking  the  stage  of  extreme  regional  elevation  are  com- 
pletely buried.  Only  an  occasional  stream  like  the  Hudson  has 
maintained  its  course  unchanged  and  has  begun  excavating  the 
channel  again.  But  even  in  this  case,  as  will  be  shown  under  a 
separate  head,  the  work  of  reexcavation  is  only  just  begun  and 
the  amount  yet  to  be  done  and  the  corresponding  original  depth 
of  the  gorge  are  wholly  unknown. 

Certain  surface  features,  however,  are  readable  and,  considered 
with  due  regard  for  all  possible  causal  factors,  give  very  useful 
suggestions.  From  them  one  obtains  clews  as  to  (i)  the  attitude 
nr  relations  of  the  hard  and  soft  beds  and  the  weak  zones,  (2) 
the  dip  and  strike  of  strata,  (3)  the  persistence  of  a  formation, 
(4)  the  occurrence  of  faults.  (5)  the  direction  of  the  chief  dis- 
turbances, (6)  the  resistance  and  durability  of  local  rock  types  — 
in  short  the  structural  characters  of  all  kinds  because  difference^ 
in  the  distribution  of  these  characters  have  given  the  different  topo- 
graphic forms  and  geographic  areas.  They  have  made  the  feature^ 
of  the  Highlands  look  different  from  those  of  the  Catskills,  and 
those  of  Wallkill  valley  different  from  the  Croton.  Because  of 
the  long  train  of  conditions  with  which  these  surface  features  arc 
each  involved  and  the  structures  that  they  indicate  they  become 
easily  the  chief  factors  in  preliminary  judgment  of  comparative 
practicability  of  rival  locations,  and  are  the  most  reliable  guide  to 
direction  and  character  and  extent  of  exploratory  investigation  for 
many  engineering  enterprises. 

d  Physiographic  zones.  Tn  summarizing  the  physiographic 
data  it  appears  that  the  following  belts  or  zones  may  be  regarded 
as  fairly  distinct  units : 


Plate  12 


GFOLOGIC 
FORMATIONS 


Catskill  and 
Oneonta  sand- 
stone conglom- 
erates 


Sherburne  flags 


Hamilton  and 
Marcellus  shales 

Onondaga  lime- 
stone 

Esopus  grit 

The  Helderberg 
series 

Shawangunk 
conglomerate 


Hudson  River 
shales,  sand- 
stones and  slates 


Wappinger  lime- 
stone 


Poughquag 
quartzite 


Storm  King 

,-granite 

The  Highlands 
"i  gneisses 


||     The  Catskill  I 
mountains 


Ashokan  ^reservoir 


Hamilton  escarp- 
ment 

Esopus  creek 

High  Falls 

Rondout  creek 

Shawangunk 
mountains 
Wallkill  river 


Hudson  river 


New  Hamburg 
Wappinger  creek 


Fish  kill  creek 
Newbn  rgh 

Breakneck 

mountain 
Storm  King 
mountain 
Bull  mountain 
Crows  Nest 
Foundry  brook 

Cold  Spring 
West  Point 


Relief  map  of  the  region  from  the  Catskill  mountains  to  the  Highlands 
showing  the  principal  physiographic  features.  (The  original  model  shows 
also  the  areal  and  structural  geology.)  (Taken  from  model  made  in  the 
physiographic  laboratory  of  Columbia  University  by  Messrs  Billingsley, 
Gnmes  and  Baragwanath) 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


73 


(1)  Coastal  plain.  A  district  underlain  by  Cretaceous  and  later 
rocks  and  confined  to  a  part  of  Staten  Island  and  Long  Island, 
not  exceeding  400  feet  relief.  This  zone  is  characterized  by  den- 
dritic drainage,  except  a  narrow  belt  on  its  inner  margin  which 
is  a  longitudinal  valley  of  the  "  inner  lowland  "  type.  Long  Island 
sound  occupies  the  position  of  this  old  adjusted  valley. 

(2)  Piedmont  belt.  A  district  lying  between  the  coastal  plain 
and  the  Highlands.  It  is  underlain  chiefly  by  crystalline  rocks  and 
metamorphosed  sediments.  Not  exceeding  800  feet  relief.  It  is 
characterized  by  adjusted  drainage  obscured  only  by  drift.  The 
ridges  and  valleys  trend  northeast  and  southwest  close  together 
and  with  very  little  variation  on  the  east  side  of  the  Hudson, 
while  on  the  west  side  the  gentle  dips  of  the  Triassic  give  broader 
and  more  unsymmetrical  forms  with  dip  slopes  and  escarpments 
wholly  independent  of  the  opposite  side.  The  zone  is  essentially 
transitional  between  the  simple  forms  of  the  coastal  plain  and  the 
complex  mountainous  character  of  the  Highlands. 

(3)  Highlands.  The  rugged  elevated  zone  formed  by  the  crys- 
talline gneisses.  Reaching  elevations  of  1600  feet.  It  is  character- 
ized by  irregular  mountain  masses  and  lofty  ridges  of  a  general 
northeast  trend  but  with  many  prominent  irregularities  both  of 
form  and  of  drainage.  The  valleys  are  deep  and  narrow.  There 
are  many  steep  escarpments.  It  is  a  mountainous  zone  in  which 
complex  structures  and  rocks  have  led  to  the  development  of  com- 
plex forms.  The  zone  forms  a  sort  of  barrier  20  miles  wide  across 
the  Hudson  river  which  exhibits  its  most  zigzag  and  narrow  and 
gorgelike  development  in  this  district. 

(4)  Appalachian  folds.  Characterized  by  folded  Paleozoic  rocks 
north  of  the  Highlands.  Reaching  elevations  of  1500  feet  rarely 
—  general  relief  400-800  feet.  North  of  the  Highlands  the  relief 
is  much  less  pronounced.  The  softer  rocks  of  the  early  Paleozoic 
formations  permitted  the  development  of  a  broad  valley  with  almost 
perfectly  adjusted  tributaries,  most  of  which  on  the  west  side  of 
the  Hudson  are  reversed.  The  topographic  forms  give  expression 
to  the  universal  folding  and  faulting  of  the  formations.  It  is 
essentially  a  transition  from  the  complex  mountain  zone  of  the 
Highlands  to  the  much  simpler  Catskill  area. 

(5)  Catskill  Monadnock  group.  Characterized  by  undisturbed 
Paleozoic  strata  and  very  strong  relief  —  reaching  elevations  of 
3500  feet.  The  eastern  margin  is  an  escarpment  facing  the  Esopus 
and  Rondout  valleys  which  are  adjusted  to  the  gently  dipping 
strata  of  that  side.    Over  the  rest  of  the  district  the  beds  lie  so 


74 


NEW   YORK  STATE  MUSEUM 


flat  that  drainage  is  essentially  dendritic  modified  slightly  hy  joint- 
ing. The  great  relief  of  the  Catskills  is  due  wholly  to  erosion 
of  flat  but  very  resistant  strata  that  withstood  the  destructive  ero- 
sion of  Cretaceous  peneplanation  and  stand  as  residuary  rem- 
nants even  to  the  present  time.  The  Catskills  are  therefore  essen- 
tially a  Monadnock  group.  In  structure  they  are  almost  as  simple 
as  the  higher  portions  of  the  cuesta  of  Long  Island,  and  they  hold 
the  same  relation  to  the  forms  developed  by  erosion  out  of  the 
old  Paleozoic  coastal  plain  of  the  interior. 

Summary 

Physiographically  the  most  complex  zone  is  midway  in  the  region 
under  discussion  —  i.  e.  The  Highlands.  This  belt  is  bordered  on 
both  sides  by  less  complicated  zones  of  less  relief,  of  more  regular 
topographic  forms  and  less  obscure  history —  the  Piedmont  cone 
on  the  south  and  the  Paleozoic  folds  on  the  north.  The  outer  mar- 
gins are  both  simple,  essentially  eroded  coastal  plains  with  strata 
dipping  away  from  the  central  belts  and  on  which  forms  and  drain- 
age lines  characteristic  of  such  history  are  developed.  These  outer 
zones  are  the  coastal  plain  of  Long  Island  on  the  south  and  the 
Catskill  Monadnock  group  on  the  north.  It  matters  little  that  they 
differ  in  age  by  almost  half  of  the  known  geologic  column. 


II 


GEOLOGIC  PROBLEMS  OF  THE  AQUEDUCT 

INTRODUCTION 

The  group  of  studies  assembled  in  this  part  are  chiefly  those  that 
have  required  considerable  exploratory  investigation  in  connec- 
tion with  the  proposed  Catskill  aqueduct  and  that  have  furnished 
new  data  of  a  geologic  character.  In  some  cases  the  additional 
investigations  have  discovered  new  and  wholly  unknown  structures 
or  conditions  and  in  all  cases  the  features  as  now  established  are 
much  more  accurately  known  than  would  otherwise  have  been 
possible. 

The  benefits  of  the  studies  have  been  twofold  and  reciprocal. 
On  the  one  side  the  practical  planning  of  the  enterprise  has  con- 
stantly required  an  interpretation  of  geologic  conditions  as  a  guide 
to  locations  and  methods  and  on  the  other  the  extensive  investi- 
gations carried  on  have  given  an  opportunity  for  practical  appli- 
cation of  geologic  principles  under  conditions  seldom  offered  and 
the  data  secured  in  additional  explorations  serve  to  make  the  detail 
of  some  of  these  complex  features  now  among  the  most  fully 
known  of  their  kind.  Examples  of  such  cases  are  (a)  the  series 
of  buried  preglacial  gorges  (as  in  the  Esopus,  and  Rondout  and 
Wallkill  and  Moodna  valleys)  and  (b)  the  completed  geologic 
cross  sections  (such  as  the  Rondout  valley,  the  Peekskill  valley, 
Bryn  Mawr,  etc.)  and  (c)  the  numerous  additions  to  the  knowledge 
of  local  rock  conditions  (such  as  that  at  Foundry  brook,  Rondout 
creek,  Coxing  kill,  Pagenstechers  gorge,  Sprout  brook,  and  others). 

Almost  every  locality  has  its  own  specific  problem  and  its  own 
peculiar  differences  of  treatment  and  interpretation  of  features. 
Nearly  all  of  the  studies  here  presented  came  to  the  attention  of 
the  writer  and  others1  in  the  form  of  definite  problems  or  questions 
involving  an  interpretation  of  geologic  factors  and  an  application  to 
some  engineering  requirement.  Some  of  these  questions,  as  is 
pointed  out  more  fully  in  part  i,  chapter  2,  are  (a)  the  location  of 

1  Professor  James  F.  Kemp  of  Columbia  University  and  W .  O.  Crosby 
of  the  Massachusetts  Institute  of  Technology  and  the  writer  constituted  the 
regular  staff  of  consulting  geologists. 

T75] 


76 


NEW  YORK  STATE  MUSEUM 


buried  channels  beneath  the  drift,  (b)  the  character  and  depth  of 
the  drift,  (c)  the  kind  of  bed  rock,  (d)  the  condition  of  bed  rock 
for  construction  and  permanence  of  tunnel,  (c)  the  underground 
water  circulation,  (/)  the  occurrence  of  folds  and  faults,  (g)  the 
position  of  weak  zones,  (/;)  the  depth  required  for  substantial  con- 
ditions, and  many  other  similar  problems. 

These  need  not  be  treated  in  their  original  form.  Indeed  many 
of  them  have  now  ceased  to  be  problems  in  any  real  sense,  for  sub- 
sequent provings  have  made  them  simple  facts,  and  wholly  new 
questions  came  to  take  their  places.  In  some  of  the  larger  prob- 
lems, however,  it  is  believed  that  a  treatment  which  involves  a  dis- 
cussion of  the  original  problem  and  the  method  of  solving  it,  to- 
gether with  the  data  thus  secured  and  the  final  interpretation  of 
geologic  features  as  now  understood  or  established  will  be  more 
instructive  than  a  mere  enumeration  of  the  collected  results. 

So  far  as  possible  each  problem  is  treated  as  a  unit  and  fully 
enough  to  be  understood  by  itself.  But  a  general  knowledge  of 
local  geology  as  outlined  in  part  i  is  assumed. 


CHAPTER  I 


GENERAL  POSITION  OF  AQUEDUCT  LINE 

Surface  topography  constitutes  the  chief  factor  in  determining 
the  general  course  of  the  aqueduct.  It  is  planned  to  control  the 
water  so  that  it  will  flow  to  New  York  city.  There  is  therefore  a 
gradual  descent  of  aqueduct  grade  from  510  feet  A.  T.  at  Ashokan 
dam  to  295  feet  at  Hill  View  reservoir.  Wherever  the  surface  of 
the  country  is  approximately  the  same  as  the  aqueduct  grade  for 
that  district  it  permits  of  the  so  called  "  cut  and  cover  "  type  of 
construction  which  is  much  cheaper  than  any  other.  Therefore, 
other  things  being  equal,  the  position  that  will  permit  the  greatest 
proportion  of  cut  and  cover  work  would  have  a  decided  advantage. 
So  it  is  possible  from  any  series  of  good  topographic  maps  to  lay 
out  trial  lines  that  are  sure  to  be  worthy  of  consideration.  The 
topographic  sheets  of  the  United  States  Geological  Survey  and  the 
maps  of  the  New  York  Geological  Survey  are  of  great  usefulness 
in  such  preliminary  work. 

But  a  little  field  examination  shows  that  there  are  many  other 
features  and  conditions  that  materially  modify  even  comparative 
cost  and  are  still  more  important  factors  in  consideration  of  per- 
manence and  safety.  Sometimes  it  is  not  apparent  that  a  course 
has  any  objectionable  features  till  considerable  exploratory  work 
has  been  done.  Likewise  a  serious  difficulty  at  one  point  may  more 
than  counterbalance  advantages  at  some  other,  so  that  considerable 
portions  of  the  line  are  finally  shifted  to  a  better  average  position. 
In  the  course  of  these  preliminary  explorations  much  valuable  data 
have  been  secured  that  now  relate  to  points  a  considerable  distance 
off  the  present  line.  The  information  has,  however,  been  necessary 
and  useful. 

One  of  the  cases  of  this  kind  where  geologic  conditions  have 
had  an  almost  controlling  influence  is  involved  in  the  choice  of 
place  of  crossing  of  the  Hudson  river.  It  has  involved  a  shift  of 
the  whole  line  between  the  reservoir  and  the  Highlands.  Diffi- 
culties encountered  in  finding  a  crossing  of  the  Esopus  also  con- 
tributed to  the  argument  favoring  a  shift  of  the  line  [see  map  of 
trial  lines  west  of  the  Hudson] .  One  of  the  points  where  explora- 
tory work  had  reached  definite  results  before  the  more  southerly 
line  was  finally  adopted  is  near  West  Hurley.    Here  wash  borings 

[77] 


78 


NEW  YORK  STATE  MUSEUM 


were  successfully  put  down  through 
the  fine  sands  and  silts  of  the 
lower  Esopus  valley  so  as  to  give 
a  fairly  acceptable  profile  of  the 
rock  floor  [see  fig.  7] .  Esopus  creek 
in  this  portion  of  its  course  follows 
the  Hamilton  shales  escarpment 
which  forms  a  steep  border  on  the 
west  side,  while  the  east  border  of 
the  valley  and  floor  are  formed, 
by  the  underlying  Onondaga  lime- 
stone. Gentle  westerly  dips  prevail 
for  both  formations,  so  that  in  the 
perfect  adjustment  reached  before 
the  glacial  invasion  a  cross  section 
would  have  shown  a  typical  unsym- 
metrical  valley — one  side  a  gentle 
dip  slope  and  the  other  a  bluff  de- 
veloped by  the  undercutting  of  the 
stream  as  it  shifted  against  the 
edges  of  the  shales. 

Results  of  exploration  show  that 
the  valley  is  filled  to  a  depth  of 
more  than  200  feet  with  silts  and 
sands  that  are  essentially  overwash 
and  glacial  lake  deposits.  The  flat 
surface  further  favors  this  explana- 
tion as  had  been  pointed  out  before 
any  explorations  were  made.  Later 
observations  in  that  portion  of  the 
Rondout  valley  which  is  a  continu- 
ation of  this  structural  feature  indi- 
cate similar  deposits  as  far  soutn 
as  the  new  line  at  Kripplebush,  10 
miles  away. 

In  this  instance  at  West  Hurley 
by  careful  measurement  of  dips  on 
the  Onondaga  limestone  and  the 
Hamilton  shales  it  was  possible  to 
estimate  the  approximate  depth  to 
which  the  Onondaga  floor  rock 
would  pass  by  the  time  the  base  of 


GEOLOGY  OF  THE  NEW   YORK  CITY  AQUEDUCT 


79 


the  escarpment  is  reached.  It  was  further  helieved  that  the  cov- 
ered portion  is  wholly  drift-filled  down  to  the  Onondaga.  It  was 
easy  therefore  to  estimate  the  approximate  profile  and  suggest  the 
point  of  greatest  probable  depth.  The  accompanying  figure  illus- 
trates the  form  and  structure  of  this  valley.  Each  valley  has  had 
in  a  smaller  way  a  similar  study  and  adjustment  of  location  of  line. 

The  final  result  is  shown  on  the  accompanying  map  which  indi- 
cates the  course  of  the  aqueduct  as  now  being  constructed. 


CHAPTER  II 


HUDSON  RIVER  CANYON 

This  is  a  special  study  of  the  Hudson  river  gorge1  based  upon 
explorations  by  borings  at  the  several  proposed  crossings.  Alto- 
gether 226  preliminary  borings  were  made  on  14  cross  sections. 
The  most  important  lines  of  borings  are  located  at  seven  different 
points  on  the  Hudson  [see  location  map].  Four  of  them  are  in  the 
vicinity  of  New  Hamburg,  lying  not  more  than  a  couple  of  miles 
north  and  south  of  that  village,  while  three  others  are  located  within 
the  Highlands.  [See  comparative  geologic  study  in  following 
chapter.]  The  chief  basis  of  information  on  all  but  one  of  these 
lines  is  the  wash  rig,  a  contrivance  as  already  pointed  out  that  gives 
rather  incomplete  data  [see  Relative  Values  of  Data,  pt  1].  On 
this  account  it  is  not  possible  to  give  the  true  bed  rock  profiles  of 
the  river  canyon  even  approximately  except  at  one  location,  i.  e. 
the  Storm  King— Breakneck  mountain  line.  An  occasional  diamond 
drill  hole  has  been  put  down  on  some  of  the  others  and  this  has 
been  done  systematically  at  the  Storm  King  location  in  a  persistent 
effort  to  determine  the  gorge  profile  and  bed  rock  condition. 

The  work  already  done  has  proven  that  in  the  Hudson  at  least 
the  wash  rig  borings  give  wholly  unsatisfactory  profiles.  The  holes 
do  not  penetrate  the  boulders  and  heavy  glacial  drift  that  is  now 
known  to  fill  the  canyon.  The  profiles,  however,  that  were  drawn 
from  this  sort  of  data  have  some  value.  They  indicate  that  bed 
rock  is  still  lower  and  that  the  finer  silts  extend  down  to  these 
depths.  In  some  places  there  is  a  heavier  filling  of  400  to  500  feet 
below  them  before  the  rock  floor  is  reached. 

Wherever  the  diamond  drill  has  succeeded  in  reaching  rock  the 
formational  identification  has  been  made  and  the  geological  cross 
section  is  a  little  more  complete.  As  a  matter  of  fact,  however,  at 
almost  every  locality  the  structural  relations  are  so  complex  or  so 
obscure  that  they  are  still  not  fully  known.  The  accompanying 
profiles  and  cross  sections  summarize  the  mass  of  accumulated  data : 

1  Kemp,  Prof.  J.  F.  Buried  Channels  beneath  the  Hudson  and  its  Tribu- 
taries. Am.  Jour.  Sci.  Oct.  1908.  26:301-23.  Some  of  the  accompanying  de- 
scriptions of  river  crossings  follow  closely  this  excellent  summary  of  Hud- 
son river  explorations  from  Professor  Kemp. 

[81] 


NEW   YORK  STATE  MUSEUM 


Fig.  9  Key  map  showing  the  locations  of  lines  of  wash  borings 
forming  the  basis  of  the  accompanying  cross  sections  of-  the  Hud- 
son above  the  Highlands 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


83 


i  Points  of  exploration1 

a  Tuff  crossing.  This  line  is  a  half  mile  above  Peggs  point. 
Wappinger  limestone  forms  the  east  bank  of  the  river  and  Hudson 
river  slates  the  western  bank.  There  seems  to  be  no  abnormal 
structural  relation  of  the  formations.  All  data  are  from  wash 
borings.    The  accompanying  section  gives  the  results. 

b  Peggs  point  line.  Peggs  point  is  2  miles  north  of  New 
Hamburg.  At  this  location  Wappinger  limestone  forms  the  east 
bank  and  Hudson  river  slates  the  west  bank  of  the  river  as  in  the 
previous  case.  The  limestone  dips  gently  westerly  while  the  slates 
have  a  variable  attitude.  This  is  a  normal  relation  and  there  is  no 
direct  evidence  of  any  great  structural  break.  A  large  number  of 
wash  borings  have  been  made  and  five  diamond  drill  holes  were 
driven,  three  of  them  in  the  river.  None  indicate  a  greater  depth 
than  223  feet,  although  there  is  a  wide  stretch,  1040  feet,  not  ex- 
plored by  the  diamond  drill.  This  space  must  contain  the  deeper 
gorge  if  one  exists  here.  From  the  known  conditions  at  the 
entrance  to  the  Highlands,  10  miles  further  down  stream,  where 
the  channel  is  known  to  be  more  than  500  feet  deeper,  it  may  be 
rather  confidently  asserted  that  a  deeper  inner  channel  does  exist  at 
this  point. 

c  New  Hamburg  line.  This  line  crosses  the  Hudson  from 
Cedarcliff  to  the  village  of  New  Hamburg.  The  river  is  narrow  — 
only  2300  feet.  There  are  no  drill  borings  within  the  river  channel, 
but  there  is  one  on  each  bank.  Both  penetrate  Wappinger  lime- 
stone first  and  then  pass  into  Hudson  river  slates  beneath.  How 
much  of  a  gorge  exists  here  is  wholly  unknown  except  in  so  far  as 
may  be  judged  from  the  wash  boring.  There  are  the  same  reasons 
for  believing  that  a  gorge  exists  as  those  noted  for  the  Peggs  point 
line. 

Structurally  this  line  is  probably  the  one  of  greatest  complexity. 
It  is  however  perfectly  clear  that  the  abnormal  position  of  the 
slates  and  limestone  on  the  east  side  of  the  river  is  caused  by  a 
thrust  fault.  A  similar  relation  of  the  slates  and  limestone  on  the 
west  side  must  be  due  to  a  like  movement,  but  whether  they  are 
separated  portions  of  the  same  structural  unit  or  of  two  adjacent 
ones  is  not  clear,  although  they  are  probably  distinct 

1  All  of  these  explorations  on  the  Hudson  river  have  been  under  the  direct 
supervision  of  Air  William  E.  Swift,  division  engineer,  in  charge  of  the 
Hudson  River  division. 


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NEW  YORK  STATE  MUSEUM 


ZOO' 


Tuff  ffff/vGS    5cc/,<,nUK  X 


Fig  io    Cross  sections  of  the  Hudson  river  north  of  New  Hamburg  and  of  Wappinger  creek 
based  upon  wash  borings.    [For  locations  see  key  map,  fig.  9] 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


85 


Five  lines  of  wash  borings  were  followed,  and  the  results  of  these 
are  indicated  in  the  accompanying  figures.  A  maximum  depth  of 
263.5  feet  is  shown  by  these  wash  borings. 

d  Danskammer  line.  This  line  is  about  a  mile  south  of  Xcw 
Hamburg.  Two  lines  of  wash  borings  were  made,  reaching  a 
maximum  depth  of  268.5  feet-  I"  this  case  slates  standing  almost 
vertical  form  the  east  bank  and  limestone  dipping  gently  eastward 
the  west  bank  of  the  river.  Whether  there  is  a  deeper  gorge  or  a 
more  complex  structure  here  is  wholly  unknown. 

Of  the  three  remaining  lines,  all  of  which  are  within  the  High- 
lands, that  one  projected  between  Storm  King  mountain  on  the 
west  and  Breakneck  ridge  on  the  east  has  been  much  the  most 
thoroughly  explored.  It  is  known  as  the  Storm  King  line.  The 
other  two  have  seemed  to  merit  less  attention.  One  crosses  the 
river  from  Crows  Nest  mountain  to  Little  Stony  point  and  Bull 
mountain  just  north  of  Cold  Spring,  and  is  known  as  the  Little 
Stony  point  line.  The  other  crosses  at  Arden  point  about  a  mile 
south  of  West  Point  and  Garrison. 

e  Arden  point  line.  Only  wash  borings  were  made.  A 
maximum  depth  indicated  by  this  method  is  220  feet.  Structurally 
this  location  appeared  to  have  disadvantages,  and  although  the 
evidence  as  to  bed  rock  conditions  is  confined  to  the  natural  out- 
crops, there  is  no  doubt  but  that  it  has  objectionable  features  of  this 
sort. 

The  Hudson  follows  closely  the  structural  control  in  this  portion 
of  its  course.  These  structural  elements  include  the  foliation,  the 
bedding  of  the  original  sediments,  the  subsequent  shearing  zones, 
and  the  strike  of  folds  and  faults.  Crushed  and  sheared  zones  are 
nowhere  in  the  Highlands  seen  so  extensively  developed  as  on  the 
islands  and  the  east  bank  of  the  Hudson  in  this,  the  central  portion 
of  its  Highlands  course.  The  river  is  very  narrow,  being  only  2120 
feet  on  this  line. 

f  Little  Stony  point  line.  The  river  here  is  2360  feet  wide. 
The  rocks  on  each  side  are  similar  and  give  no  clue  to  possible 
depths  of  channel.  Less  than  200  feet  was  reached  by  the  lines  of 
wash  borings.  Three  drill  borings  penetrated  the  stony  or  bouldery 
river  filling  somewhat  deeper  —  one  near  the  center  reaching  322 
feet.   None,  however,  reached  bed  rock. 

g  Storm  King  crossing.  Extensive  exploratory  work  has 
been  carried  on  at  this  point,  both  on  the  banks  and  in  the  river. 
Wash  borings  as  usual  have  given  poor  results.  Two  diamond  drill 
holes  were  run  at  an  angle  toward  and  beneath  the  margins  of  the 


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i  P£Ges  Po/a/t  5o</TH  /?f?AI6£  Section  O.P  J 


Fig.  II    Cross  sections  of  the  Hudson  river  near  New  Hamburg  based  on  wash  borings 
[For  locations  see  key  map,  fig  9] 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


87 


river,  and  in  addition  a  working  shaft  suitable  for  permanent  use 
has  been  started  on  each  side  of  the  river.  These  have  thoroughly 
explored  the  rock  character  to  a  depth  of  about  800  feet,  it  has 
proven  to  be  of  constant  type,  a  gneissoid  granite,  affected  by 
moderate  amount  of  jointing,  shear  movements  and  occasional  dike 
intrusion.  The  two  sides  are  alike,  the  rock  in  depth  is  com- 
paratively free  from  water,  nearly  all  coming  from  the  adjacent 
surface  drainage. 

Persistent  efforts  have  been  made  to  use  the  drill  in  the  river  to 
explore  the  rock  channel,  but  with  meager  results.  The  difficulties 
to  be  overcome  in  drilling  in  this  tidal  river  to  the  necessary  depth 
are  probably  greater  than  have  even  been  encountered  in  any 
similar  undertaking.  The  disturbance  presented  by  the  current,  the 
tide,  the  depth  of  water,  the  drift  filling  above  the  rock  channel, 
and  the  traffic  in  the  river  are  a  constant  menace.  The  complex 
character  of  drift  filling  in  this  gorge,  especially  the  occasional 
heavy  bouldery  structure,  makes  it  necessary  to  reduce  the  size  and 
recase  the  holes  repeatedly.  But  in  this  regard  the  work  has 
suffered  less  actual  loss  than  by  the  menace  of  river  traffic. 
Several  times  after  the  greatest  efforts  had  been  put  forth  in 
pushing  the  drills  deep  into  the  gorge  a  helpless  or  unmanageable  or 
carelessly  guided  steamer  or  scow  has  wrecked  the  work.  In  this 
way  some  of  the  most  critical  locations  have  been  lost  together  with 
many  months  of  labor. 

The  results  are  shown  on  the  accompanying  drawings. 

It  is  worth  noting  that  of  those  holes  located  far  out  in  the  river 
channel  only  two  have  reached  bed  rock.  Even  these  two  have 
penetrated  the  rock  so  little  distance  that  there  might  be  still  some 
doubt  of  permanent  bed  rock.  The  fact,  however,  that  the  rock 
found  is  of  the  right  type,  i.  e.  like  the  walls  of  the  gorge,  leads  to 
the  conclusion  that  the  bottom  was  actually  penetrated.  Neither  of 
these  holes  are  in  the  middle  of  the  river,  and,  although  the 
maximum  depth  of  608  feet  was  reached  by  one  of  them,  the  central 
portion  of  the  buried  channel  proves  to  be  still  deeper.  One  hole 
located  near  the  middle  was  able  to  penetrate  to  a  depth  of  626  feet 
without  striking  bed  rock.  But  it  was  finally  lost.  The  latest 
results  are  from  a  boring  that  has  reached  a  total  depth1  (January 
1,  1910)  of  703  feet,  the  last  8  feet  of  which  was  believed  by  the 
drillers  may  be  in  bed  rock.    All  above  is  drift  and  silt. 

1  Subsequent  exploration  has  proven  that  the  bottom  of  the  old  channel 
lies  still  deeper.  This  boring  has  been  pushed  to  a  depth  of  751  feet  with- 
out yet  touching  bed  rock  (Oct.  8,  1910). 


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NEW  YORK  STATE  MUSEUM 


Jrig.  12    Cross  sections  of  the  Hudson  river  at  four  points  between  Danskammer  Light  and 
New  Hamburg  [see  key  map,  fig.  9,  for  locations] 


ft 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


89 


2  Discussion 

The  present  facts  therefore  indicate  that  the  buried  Hudson 
channel  is  more  than  700  feet  deep  between  Storm  King  and 
Breakneck  ridge.  Furthermore  this  is  more  than  twice  as  great 
depth  as  has  been  found  (so  far  as  yet  tested)  at  any  other  point 
either  above  or  below  this  place.  Although  data  of  this  kind  are 
scarce  yet  there  are  two  other  borings  that  have  given  surprising 
results — -(a)  at  Peggs  point  and  (/?)  the  Pennsylvania  borings  at 
New  York  city. 

Peggs  point.  At  this  place,  where  studies  were  made  for  a 
possible  crossing,  a  hole  700  feet  from  shore  struck  rock  at  223 
feet  and  the  unknown  space  or  interval  within  which  it  is  possible 
for  a  channel  to  lie  is  less  than  1040  feet  wide.  This  is  about  10 
miles  above  the  Storm  King  crossing  and  in  much  softer  rock 
(Hudson  River  slates).  Yet  the  Storm  King  gorge  in  granite  is 
deeper  than  that  (deeper  than  223  feet)  for  a  width  of  nearly  2500 
feet.  Of  course,  there  may  be,  and  probably  there  is,  a  much 
deeper  channel  at  Peggs  point  within  the  1040  feet  unexplored 
space.  But  even  so  there  is  a  remarkable  discrepancy  in  width  of 
gorge  at  these  two  points  that  must  be  accounted  for  in  some  other 
way  than  simple  stream  erosion. 

The  Pennsylvania  borings  opposite  33d  st.,  New  York  city. 
The  data  gathered  by  the  Engineers  of  the  Pennsylvania  Tunnel 
Company  in  their  explorations  for  tunnel  from  33d  street,  Man- 
hattan, to  Jersey  City,  have  recently  been  made  public.  There  are 
six  holes  into  rock.  Their  positions  and  depth  to  rock  bottom  are 
given  below : 

a  800'  from  New  York  bulkhead  190'  to  bed  rock  =  aplite 
b  1000'  from  New  York  bulkhead  290'  to  bed  rock  =  hornblende 
schist 

c  2180'  from  New  York  bulkhead  300'  to  bed  rock  =  chloritic 
and  serpentinous  rock. 

d  2350'  from  New  York  bulkhead  26o'(  ?)  to  probable  boulder  = 
jasper  breccia 

c  3300'  from  New  York  bulkhead  270'  to  bed  rock  =  arkose 
sandstone 

/  13700'  from  New  York  bulkhead  225'  to  rock=brown  sand- 
stone 

There  are  other  shallower  borings  on  both  sides  of  the  river. 
Those  on  the  Manhattan  side  are  represented  by  several  different 
facies  of  Manhattan  mica  schist  and  granite  and  pegmatite  in- 


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NEW  YOUK  STATE  MUSEUM 


trusives,  while  the  New  Jersey  side  is  represented  by  different 
varieties  of  arkose  and  gray  and  brown  sandstone  belonging  to  the 
Newark  series. 

It  should  be  noted  that  although  only  one  hole  marks  rock  bottom 
as  low  as  300'  (that  one  situated  2180'  from  the  New  York  bulk- 
head about  the  middle  of  the  river),  yet  there  is  at  least  a  1100  foot 
space  on  each  side  which  is  essentially  unexplored,  and  within  one 
of  these  spaces  there  may  be  a  deeper  gorge. 

The  cores  taken  from  the  east  side  of  this  middle  zone  belong  to 
facies  of  the  Manhattan  schist  formation,  while  those  on  the  west 
side  .belong  to  the  Newark  series.  The  middle  one,  however,  is 
essentially  a  soapstone  or  serpentine  and  may  be  a  continuation  of 
the  Hoboken  serpentine  belt.  In  any  case,  it  belongs  in  age  to  the 
older  series  of  formations. 

It  is  certain  that  here  again,  50  miles  below  Storm  King  locality, 
a  very  deep  gorge,  if  one  exists,  must  be  comparatively  narrow. 

Submarine  channel.  It  is  worth  noting  in  this  same  connec- 
tion that  a  submerged  gorge  has  been  mapped  by  the  Coast  and 
Geodetic  Survey  on  the  continental  shelf  from  the  vicinity  of 
Sandy  Hook  to  the  deep  sea  margin,  a  distance  of  more  than  a 
hundred  miles.  This  is  interpreted  by  Spencer1  and  others  with 
apparently  sound  argument  as  the  lower  portion  of  the  old  pre- 
glacial  Hudson  gorge  formed  during  an  epoch  of  great  continental 
elevation.  The  outer  portion  of  this  submerged  gorge  is  very  deep. 
That  section  near  shore  is  shallow  and  obscure.  It  has  been 
assumed  that  this  obscurity  and  shallowness  is  due  to  offshore  and 
river  deposition,  filling  the  channel  with  silt.  No  better  explanation 
is  yet  forthcoming.  But  even  here  the  width  of  the  submerged 
gorge  is  suggestive.  In  very  much  softer  sediments  than  any  en- 
countered in  its  whole  course  on  present  land,  and  in  a  part  of  its 
course  from  50  to  100  miles  below  the  other  sections,  the  river  has 
cut  a  gorge  only  4000  feet  wide  at  top  and  2000  feet  deep  within 
a  broader  valley  5  miles  wide.  In  its  deepest  known  part  the 
proportions  are  10,000  feet  in  width  at  top  to  3800  feet  in  depth. 

From  this  it  would  appear  that  the  inner  gorge  type  of  develop- 
ment is  characteristic  of  the  Hudson,  and  that  it  was  originally  an 
exceedingly  narrow  one  compared  to  the  present  river  width,  indi- 
cating rapid  erosion  during  a  brief  and  comparatively  recent  epoch. 
This  submerged  continental  margin  condition  is  favorable  to  the 


1  Spencer,  J.  W.  The  Submerged  Great  Canyon  of  the  Hudson  River 
Am.  Jour.  Sci.  1905,  v.  19. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


91 


assumption  that  there  are  narrower,  still  deeper  channels  within  the 
unexplored  spaces  both  at  New  York  city  and  at  Peggs  point. 

The  only  known  exception  and  the  one  really  surprising  section 
is  the  Storm  King  crossing.  It  is  too  wide,  considering  the  profiles 
at  Peggs  point  and  at  New  York  city  for  simple  normal  stream 
erosion.  That  is  clear  enough.  But  a  still  more  difficult  question  is 
whether  it  is  also  too  deep.  It  is  much  deeper  than  any  known 
section  above  or  below  for  a  distance  of  50  miles. 

There  appears  to  be  only  one  satisfactory  explanation  of  this 
abnormal  width  of  the  deeper  section  and  that  is  by  glacial  erosion. 
Just  above  Storm  King  is  the  wide  bay  opposite  Cornwall  and 
Newburgh.  The  few  glacial  scratches  observed  trend  about  s.  150  e. 
The  ice  therefore  moved  to  the  east  of  south,  and  it  is  noted  that  the 
course  of  the  river  is  about  the  same.  The  northern  front  of  Storm 
King  mountain  is  steep  and  trends  east  and  west  while  the  northern 
front  of  Breakneck  mountain  trends  southwest.  It  would  appear 
therefore  .that  these  slightly  converging  mountain  fronts  served  as 
sort  of  a  funnel  into  which  the  ice  was  forced  from  the  wide  gather- 
ing ground  immediately  above,  and  through  which  there  may  have 
been  a  tongue  or  stream  of  ice  of  more  than  average  power  and 
efficiency  moving  almost  in  direct  line  of  the  present  course  of  the 
river.  It  is  reasonable  to  expect  that  these  conditions  would  favor 
more  than  average  glacial  erosion. 

3  Storm  King-Breakneck  mountain  profile 

It  is  practically  impossible  to  draw  a  complete  profile  for  the 
Hudson  river  gorge  at  any  point  in  its  lower  course.  Even  at  Storm 
King  mountain  or  New  York  city  or  at  Peggs  point,  at  each  of 
which  places  considerable  exploratory  work  has  been  done,  only  the 
broadest  features  are  known.  Nevertheless,  several  things  have 
been  proven  and  they  are  worth  considering  in  this  question.  They 
may  be  summarized  as  follows : 

a.  If  there  is  a  very  deep  gorge  at  Peggs  point  (deeper  than  250 
feet)  it  can  not  be  over  1000  feet  wide. 

b  If  there  is  a  very  deep  gorge  at  New  York  city  (deeper  than 
300  feet)  it  can  not  be  over  1200  feet  wide. 

c  At  Storm  King,  located  between  the  other  two  and  in  harder 
rock  than  either  of  them,  a  gorge  at  least  400  feet  deep  is  proven 
to  have  a  width  of  more  than  1500  feet. 

It  is  certain  that  simple  stream  erosion  could  not  account  for 
such  a  difference  of  cross  section.    There  is  no  doubt  but  that  en- 


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GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


93 


larging  by  ice  so  far  as  widening  is  concerned  is  practically  proven. 
If  may  also  be  overdeepened,  by  which  is  meant  that  it  may  have 
been  gouged  out  deeper  than  could  have  been  done  by  a  stream  of 
water  alone. 

If  ice  action  then  be  granted,  the  profile  ought  to  be  and  prob- 
ably is  essentially  an  ice  valley  profile,  i.  e.  of  a  more  or  less  U- 
shape,  rather  than  of  typical  stream  erosion  form.  It  is  certain 
also  in  this  case,  if  glacial  overdeepening  is  admitted,  that  there  can 
be  no  stream  notch  in  the  bottom  of  it.  The  significance  of  this 
lies  in  the  probability  that  the  floor  is  approximately  the  same  level 
on  a  considerable  portion  of  the  bottom,  so  that  when  once  the 
margin  of  this  floor  is  touched  the  gorge  as  a  whole  is  thereby 
determined  for  depth. 

After  plotting  the  borings  data  and  relying  upon  the  factors  that 
seem  to  be  most  firmly  established,  it  appears  that  the  following 
statements  are  as  definite  as  the  facts  will  warrant : 

a  The  average  slope  of  the  Storm  King  side  of  the  valley  above 
river  level  is  nearly  380,  and  this  is  in  several  steps  or  sections  of 
steeper  and  flatter  slopes.    The  Breakneck  side  is  about  the  same. 

b  The  average  slope  of  the  Breakneck  side  of  the  gorge  below- 
present  water  level  (the  side  on  which  alone  there  are  enough  data 
to  plot  a  fairly  good  curve)  does  not  vary  much  from  this  same 
value  [see  accompanying  profile] .  And  it  is  also  in  steeper  and 
gentler  slopes,  apparently  a  series  of  U-shaped  forms  set  one  inside 
the  other,  each  inner  one  deeper  than  the  next  outer  one.  Each  suc- 
cessive inner  step  is  approximately  300  feet  deeper  than  the  last 
and  1000  feet  narrower. 

It  is  certain  that  this  sort  of  profile  is  not  as  simple  as  at  first 
appears.  The  surprising  feature  is  the  close  approximation  of  the 
slopes  above  and  below  present  river  level.  In  view  of  the  fact 
that  glacial  widening  has  been  practically  proven,  as  shown  before, 
not  much  importance  can  be  attached  to  this  uniformity  or  simi- 
larity of  slope.  Ordinarily  such  a  persistence  of  slope  would  be 
taken  to  indicate  simple  stream  origin,  but  having  abandoned  that 
hypothesis,  the  value  of  the  angle  as  a  factor  in  estimating  prob- 
able total  depth  is  lost.  In  short,  one  can  not  assume  that  the 
deepest  point  is  indicated  by  the  intersection  of  the  slopes  of  the 
two  sides. 

But  there  is  one  feature  that  is  at  least  suggestive.  That  is  the 
uniformity  of  the  succession  of  steps  and  slopes.  It  was  noted 
above  that  each  successive  inner  one  is  about  300  feet  deeper  and 
1000  feet  narrower.    If  this  uniformity  and  proportion  is  main- 


GEOLOGY  OF  THE  NEW   YORK  CITY  AQUEDUCT 


95 


tained  for  the  next  inner  one  —  inside  of  holes  no.  10  and  no.  22 
—  there  would  be  room  for  only  one  more  and  its  approximate 
depth  would  lie  somewhere  between  800  feet  and  900  feet  below 
tide. 

Recent  drilling  has  shown  a  marked  difference  between  holes 
no.  10  and  no.  22.  Hole  no.  10  located  500  feet  southeast  of  no. 
22  is  nearly  100  feet  deeper.  Since  no.  10  is  nearly  straight  down 
stream  this  discrepancy  is  disturbing.  But  if  one  considers  the 
distance  of  each  from  the  east  bank  it  is  noted  that  no.  10  is  900 
feet  out  and  no.  22  is  800  feet.  Hole  no.  10  is  thus  about  100 
feet  nearer  the  middle  of  the  stream  and  allowing  for  this  addi- 
tional distance  according  to  the  profile  as  known  it  ought  to  be 
at  least  70  feet  deeper  than  no.  22.  This  corrected  difference  then 
of  30  feet  does  not  seem  to  be  of  much  importance. 

Summary.  Everywhere  in  its  lower  course  the  Hudson  ex- 
hibits the  character  of  a  narrow  gorge,  sometimes  of  a  gorge  within 
a  gorge,  most  of  which  is  either  submerged  or  buried  several  hun- 
dred feet. 

Depths  of  200  to  300  feet  are  average  and  for  the  last  60  miles 
of  its  course  represent  widths  of  1000  to  3000  feet. 

Greater  depths  are  believed  to  be  maintained  continuously  within 
a  narrower  inner  notch,  but  of  this  there  is  no  conclusive  proof 
and  very  little  evidence  outside  of  a  few  Storm  King  borings. 

The  Storm  King-Breakneck  notch  is  over  751  feet  deep.  But 
it  is  abnormal  at  least  in  width  and  probably  also  in  depth,  due  to 
ice  erosion. 

The  conditions  indicate  ( a )  rapid  stream  erosion  while  the  con- 
tinent stood  much  higher  than  now,  (b)  glaciation  which  enlarged 
the  gorge  in  at  least  a  few  places  and  filled  it  with  rock  debris 
and  later  with  mud  during  submergence,  (c)  finally  an  emergence 
with  minor  oscillations  and  erosion  to  the  present  time. 

4  Origin  of  the  present  course  of  the  Hudson 

The  course  of  the  Hudson  is  in  most  respects  no  more  abnormal 
than  that  of  the  Susquehanna.  Both  flow  across  mountain  ridges 
in  such  manner  as  to  indicate  their  superimposed  character.  Both 
date  back  to  the  Cretaceous  peneplain.  But  the  striking  feature 
of  the  Hudson  is  its  straight  course.  As  Hobbs  and  others  have 
pointed  out,  the  river  is  abnormally  straight  for  more  than  200 
miles  —  and  this  in  spite  of  the  fact  that  it  crosses  the  bedding  and 
other  structures  of  the  country  rock  at  nearly  all  points  at  an 


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NEW   YORK  STATE  MUSEUM 


oblique  angle.  Such  conditions  are  especially  notable  south  of  the 
Highlands  where  the  Hudson  cuts  at  a  low  angle  across  the  ends 
of  a  succession  of  complex  folds  of  the  crystalline  metamorphics 
for  30  miles  to  New  York  city.  But  this  is  true  only  of  the  east 
side  of  the  river.  The  west  bank  is  an  almost  unbroken  uniform 
escarpment  of  the  Palisade  diabase  intruded  sheet  underlain  by 
Newark  sandstones,  which  if  laid  down  upon  a  pretty  well  planed 
Pretriassic  surface  might  easily  control  the  Hudson,  and  which 
would  not  differ  from  its  present  course. 

The  most  evident  exception  to  this  is  the  course  of  the  river 
from  Ploboken  to  Staten  Island.  Instead  of  following  the  line  of 
contact  between  the  crystallines  and  Triassic  formations,  the  river 
cuts  through  the  crystallines  leaving  large  masses  of  serpentine 
and  associated  schist  on  the  west  side.  This  together  with  the 
behavior  of  the  river  in  cutting  across  the  strike  farther  north 
near  the  Highlands  is  believed  to  strongly  favor  the  fault  theory 
of  location  especially  south  of  the  Highlands.  The  same  condi- 
tions would  be  favorable  to  the  development  of  a  narrow  gorge 
and  perhaps  a  very  deep  one  rapidly  eroded  along  the  crush  zone 
of  the  fault. 

From  the  northern  entrance  to  the  Highlands  to  Haverstraw  bay, 
where  the  Palisades  arc  reached,  the  stream  course  is  not  by  any 
means  straight,  but  shifts  from  longitudinal  structure  to  cross 
structure  alternately  in  a  zigzag  manner.  North  of  the  Highlands 
the  course  is  more  direct  again.  On  the  whole  the  present  explora- 
tions have  added  little  to  the  facts  bearing  upon  this  question. 
Faults  crossing  the  river  arc  common  and  easily  recognized.  Oc- 
casionally one  appears  to  pass  into  the  river  gorge  at  a  very  small 
angle  and  not  reappear.  In  a  few  places,  especially  in  the  High- 
lands, the  course  does  not  seem  to  be  consistent  with  the  hypothe- 
sis of  a  large  fault  line.  It  is  to  be  expected  that  further  work  at 
the  Hudson  river  crossing  will  add  materially  to  ithe  facts  relating  to 
the  structures  within  the  gorge. 


CHAPTER  III 


GEOLOGICAL   CONDITIONS  AFFECTING  THE  HUDSON 
RIVER  CROSSING 

General  statement 

This  is  essentially  a  study  of  the  geologic  features  and  condi- 
tions shown  by  exploration  to  have  an  important  influence  upon 
the  choice  of  river  crossing  for  the  aqueduct.  In  the  beginning  it 
was  possible  to  consider  that  any  point  between  Poughkeepsie  and 
New  York  might  furnish  a  crossing.  The  early  preliminary  in- 
vestigations showed  that  it  would  be  desirable  to  cross  either  above 
or  within  the  Highlands  and  subsequent  exploratory  work  throws 
light  on  different  possible  locations  in  these  regions.  Fourteen  dif- 
erent  lines  were  tested  by  wash  borings.  Later  some  of  these  were 
tested  by  diamond  drill.  As  data  accumulated  it  was  possible  to 
eliminate  many  of  the  trial  lines  and  the  more  detailed  and  critical 
studies  became  confined  to  a  few  important  possible  crossings. 

In  making  a  comparison  of  them  as  to  geological  environment  it 
is  evident  that  they  fall  into  two  distinct  groups1  [see  fig.  15]- 
One,  that  may  be  designated  the  "  New  Hamburg  "  group  is  rep- 
resented by  the  "  Peggs  point,  '  "  New  Hamburg,"  and  "  Dan- 
skammer  "  lines  and  is  characterized  by  a  series  of  much  folded, 
faulted  and  crushed  sedimentary  rocks,  chiefly  slates,  limestones 
and  quartzites.  The  other,  that  may  be  called  the  Highlands  group, 
is  represented  by  the  "  Storm  King,"  "  Little  Stony  point,"  and  the 
"Arden  point  "  lines  and  is  characterized  by  crystalline  metamor- 
phic  and  igneous  rock  of  a  much  older  series. 

A  judgment  as  to  the  most  desirable  crossing  involves  the  selec- 
tion of  one  of  these  groups  chiefly  upon  general  geologic  features, 
and  finally  a  selection  of  a  particular  line  upon  minor  differences  of 
materials  or  structure. 

In  the  first  place  it  seems  necessary  to  consider,  for  each  group, 

1  There  have  been  other  suggestions  for  crossing  the  Hudson  river, 
farther  upstream  and  farther  down  than  these  —  one  being  at  New  York 
city  —  but  none  have  had  sufficient  claim  to  attention  to  encourage  much 
detailed  work  or  so  careful   consideration    as  those  here  discussed. 

A  shift  of  position  of  the  Hudson  river  crossing  involved  a  correspond- 
ing shift  of  a  large  section  of  the  northern  aqueduct  line.  The  first  choice 
of  location  occasioned  a  shift  southward  of  all  that  portion  between 
Ashokan  reservoir  and  the  Hudson. 

[97] 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


99 


the  whole  length  of  pressure  tunnels  whose  position  would  be  modi- 
fied by  a  shifting  of  river  crossing.  This  is  because  the  aqueduct 
will  approach  the  Hudson  with  nearly  400  feet  head  —  i.  e.  400 
feet  above  river  level  or  with  an  equivalent  pressure.  For  this 
reason  it  is  considered  necessary  to  plan  a  rock  pressure  tunnel 
beneath  the  river  which  can  deliver  the  water  at  nearly  the  same 
elevation  again  on  the  east  side. 

Thus  any  one  of  the  "  New  Hamburg  group  "  involves  a  contin- 
uous pressure  tunnnel  reaching  from  the  margin  of  Marlboro  moun- 
tain to  Fishkill  range,  a  distance  of  approximately  seven  miles, 
while  any  of  the  "  Highlands  group  "  permits  the  substitution  of 
two  more  or  less  separate  siphon  tunnels  (Moodna  creek  and  Hud- 
son river)  of  considerably  less  combined  total  length. 

A  reliable  conclusion  as  to  the  choice  of  crossing  is  probably  best 
reached  through  a  comprehensive  understanding  of  the  geologic  de- 
velopment of  the  region  together  with  a  consideration  of  specific 
local  conditions.  With  this  end  in  view  a  condensed  outline  of 
geologic  history,  so  far  as  it  bears  upon  the  questions  at  issue,  is 
inserted.  But  for  a  more  comprehensive  discussion  of  these  matters 
the  reader  is  referred  to  the  explanatory  chapter  of  part  1. 

Geology 

This  particular  locality,  including  as  it  does  the  Highlands  of 
the  Hudson  and  the  district  lying  along  its  northern  border,  is  one 
of  the  most  complicated  stratigraphically  and  structurally  to  be 
found  in  the  entire  region.  The  strata  represented  include  more 
than  half  the  total  geologic  scale  reaching  from  the  oldest  sedi- 
ments following  the  Archean  up  to  and  including  a  part  of  the 
Devonic  series  [see  pt  1].  The  rock  types  include  granites,  diorites, 
gneisses,  schists,  marbles,  serpentines,  slates,  quartzites,  sandstones, 
limestones,  shales,  and,  less  extensively,  other  varieties.  And  the 
region  bears  the  evidence  of  no  less  than  three  periods  of  mountain- 
making  disturbances,  each  in  its  turn  adding  to  the  succession  of 
foldings,  faultings  and  unconformities. 

The  oldest  formation  is  a  crystalline  gneiss  —  a  characteristic 
rock  of  the  Highlands.  It  represents  an  ancient  sediment  that  has 
been  completely  recrystallized  during  some  of  the  earlier  mountain- 
making  period.  It  is  older  than  the  Cambric.  Interbedded  with 
it  to  a  limited  extent  are  quartzite  beds,  ancient  limestones  (now 
usually  serpentinous  in  character)  and  schistose  beds ;  and  in  it  are 
many  igneous  injections,  mostly  granites  of  various  types.  All 

4 


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NEW  YORK  STATE  MUSEUM 


these  igneous  injections  are  therefore  younger  than  the  gneiss  and 
are  very  large  and  abundant  in  certain  cases.  The  granite  of  Storm 
King,  Crows  Nest  and  Breakneck  ridge  belongs  to  this  type. 

Following  the  sedimentary  cycle  represented  by  the  above  series, 
and  perhaps  others  not  now  preserved,  the  region  was  folded  into 
a  mountain  range,  the  series  was  extensively  metamorphosed  and 
passed  through  a  long  period  of  erosion  during  which  it  was  again 
reduced  to  sea  level  position  and  began  to  accumulate  a  new  series 
of  sediments. 

The  lowest  beds  occurring  upon  this  foundation  are  sandstones, 
now  changed  into  quartzite.  In  places  they  are  conglomeritic,  and 
may  now  be  seen  projecting  into  the  valleys  along  the  Highland 
border.  This  formation  is  of  Cambric  age,  and  is  from  200  to  600 
feet  thick  in  favored  places.  It  forms  an  almost  continuous  belt 
along  the  north  side  of  the  Highlands  except  where  cut  out  by 
faulting,  and  extends  with  similar  breaks  beneath  the  later  sedi- 
ments northward.   This  quartzite  is  known  as  the  "  Poughquag." 

Upon  the  quartzite  of  this  series  there  was  developed  a  succes- 
sion of  limestone  beds  at  least  900  to  1000  feet  in  thickness.  This 
formation  is  known  as  the  "  Wappinger  "  and  includes  some  beds 
that  are  of  Cambric  but  for  the  most  part  of  Ordovicic  age. 

The  final  member  of  this  series  is  a  shale  and  shaly  sandstone 
in  places  changed  to  slate.  It  is  quite  variable  in  actual  character 
and  has  a  great  thickness,  never  yet  successfully  estimated,  but 
probably  several  thousand  feet.  This  is  the  so  called  "  Hudson 
River  slate  "  series.    In  this  region  they  are  of  Ordovicic  age. 

This  is  the  succession  which  the  proposed  Hudson  river  lines 
has  to  penetrate  in  a  pressure  tunnel.  Later  Siluric  and  Devonic 
strata  lie  in  the  immediate  vicinity  of  this  alternative  line,  but  add 
no  complication  to  the  problem  as  it  now  stands.  Therefore  no 
other  formations  need  be  considered  except  the  glacial  drift.  This 
covers  almost  every  rock  surface  and  is  deeply  accumulated  in 
some  places,  notably  in  the  narrow  gorges  and  valleys,  obscuring 
the  finer  original  topographic  lines. 

A  summary  of  the  history  of  the  formations  chiefly  involved  in 
this  problem  with  a  suggestion  of  later  erosion  activities  may  be 
tabulated  as  follows : 

r  Glaciation 
Reelevation 
Erosion  (interrupted) 
Elevation  (rejuvenation) 


Cenozoic 

•tlJ 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


IOI 


Afesozoic 


'  Erosion  to  peneplain 

 Unconformity 

A  long  interval  including  two  mountain-making  epochs 
and  at  least  one  period  of  general  sedimentation 


Paleozoic 


Ordovicic  ^  ^u<^son  River  slates 
|  Wappinger  limestone 
Cambric    j  Poughquag  quartzite 

 Unconformity 

A  long  interval  including  mountain  folding,  igneous 
injection,  erosion,  and  perhaps  other  sedimentations 


'  The  metamorphosed  schists,  limestones,  quartzites 
etc.,  together  with  accompanying  intruded  igneous 
masses  —  forming  the  basal  gneisses  of  the  High- 
lands 


The  evidence  of  such  succession  and  history  gathered  from  the 
scattered  outcrops  of  rock  in  the  immediate  area,  is  nowhere  better 
shown  than  in  the  field  covered  by  this  investigation. 


Structure 

When  such  outcrops  as  are  known  are  plotted  and  organized,  sev- 
eral important  facts  become  clear. 

1  The  folds  run  with  remarkable  persistence  northeast  and  south- 
west. 

2  The  succession  in  many  places  is  not  normal.  Often  a  whole 
formation  or  even  two  of  them  are  missing  and  formations  that 
should  be  separated  are  brought  side  by  side.  Faulting  therefore  is 
prevalent  and  the  occurrences  show  that  these  large  fault  lines 
usually  run  northeast  and  southwest. 

3  A  consideration  of  the  dips  of  the  strata  shows  that  most  of  the 
folds  are  overturned  as  if  pushed  by  some  general  movement  from 
the  southeast. 

4  This  same  movement  causes  the  faulting  to  be  largely  of  the 
overthrust  type,  and  in  some  cases  the  lateral  displacement  attained 
in  this  way  may  possibly  be  several  thousand  feet. 

5  Isolated  "  islands  "  of  the  older  rock  formation  appear  out  in 
the  later  sedimentary  area.  They  all  seem  to  belong  to  prolonga- 
tion of  the  ranges  of  the  Highlands  and  their  abundance  undoubt- 


102 


NEW  YORK  STATE  MUSEUM 


edly  complicates  the  underground  structure  throughout  a  consider- 
able belt.  / 

6  The  Highlands  area  terminates  in  a  serrate  margin  which,  in 
the  latest  thrust  movements  from  the  southeast,  must  have  created 
very  unequal  distribution  of  stresses  within  the  slate-limestone 
region  to  the  north  causing  additional  cross  folding  and  faulting. 
For  the  most  part  these  can  be  traced  only  a  short  distance  before 
losing  their  identity. 

In  a  mountain  folding  movement,  the  uppermost  rocks  are  most 
broken  and  displaced  or  crushed  while  those  of  greater  depth  may 
be  bent  or  uniformly  folded  or  even  recrystallized.  It  would  ap- 
pear that  this  latter  was  the  condition  of  the  Highlands  rock  series 
during  its  earlier  history.  And  even  in  the  latest  movements  its 
lines  appear  to  be  less  radically  disturbed  than  the  slates  and  lime- 
stones to  the  north.  Most  of  the  disturbances  that  invite  serious 
consideration  belong  to  the  latest  period  of  these  mountain-making 
upheavals. 

Comparison  of  routes 
i  New  Hamburg  group.  This  group  of  crossings  is  in  the 
later  sedimentary  series.  Hudson  River  slates  and  Wappinger 
limestone  are  the  chief  formations.  But  within  the  southern  third 
of  the  tunnel,  at  least,  the  underlying  Cambric  quartzite  and  the 
older  Highland  gneiss  would  be  cut  —  the  quartzite  possibly  three 
times.  The  succession  therefore  will  be  of  considerable  complexity 
as  a  whole. 

All  of  the  formations  involved  are  thrown  into  very  steep  dips 
at  most  places  and  are  consequently  liable  to  rapid  and  unexpected 
changes  —  some  of  which  probably  do  not  show  at  the  surface. 

There  are  several  fault  lines  belonging  to  the  major  northeast 
and  southwest  series  to  be  crossed  by  such  a  tunnel  —  one  of  them 
in  each  case  being  met  at  considerable  depth  and  beneath  or  adja- 
cent to  the  river.  These  faults  besides  being  the  weakest  zones  of 
rock  as  a  rule,  are  in  addition  the  most  unstable  in  any  possible 
future  earth  movements.  Although  there  is  no  evidence  of  recent 
displacement  along  these  lines,  still  such  a  thing  is  always  possible 
and  recent  serious  effects  of  this  kind  on  the  Pacific  coast  suggest 
caution.  It  is  manifestly  advisable,  if  possible,  from  every  stand- 
point to  avoid  crossing  several  of  them. 

In  the  field  there  are  numerous  springs  of  very  large  flow  along 
many  of  the  limestone  borders.  The  concentration  of  them  to 
these  situations  in  addition  to  the  occurrence  of  an  occasional  sink- 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


I03 


hole,  leads  to  the  conclusion  that  they  are  more  intimately  depend- 
ent upon  the  limestone  structure  for  their  existence  than  upon  the 
glacial  drift  or  any  superficial  factor.  Their  abundant  flow,  some- 
times on  high  ground,  indicates  rather  extensive  structural  con- 
nections and  this  is  believed  to  be  the  limestone  bed  itself  and  that 
such  flows  would  be  encountered  also  in  depth.  The  occurrence  of 
sinkholes  suggest  also  possible  solution  channels  and  cavities  and 
distant  outlets.  The  types  of  rock  to  be  encountered  on  the  lines 
represented  by  this  group  are  easily  workable.  Among  them  all 
the  Hudson  River  slates  is  probably  the  most  satisfactory  from  any 
standpoint.  It  is  generally  easy  to  penetrate  and  has  a  capacity  for 
healing  its  own  fractures.  For  this  reason  it  can  be  considered  good 
ground,  tight  and  safe.  But  a  considerable  distance  of  the  tunnel 
can  not  be  kept  in  slate  —  perhaps  even  more  of  it  than  can  be 
proven  from  surface  observations.  The  other  formations  are  con- 
siderably less  satisfactory.  The  limestones  are  in  places  shattered 
and  are  liable  to  abundant  flow  of  water.  The  quartzite  is  ex- 
tremely hard,  as  difficult  to  penetrate  as  granite,  and  where  crossed 
by  the  faults  is  probably  not  healed  at  all,  while  the  gneiss  is  doubt- 
less of  similar  character  to  that  of  the  Highlands  crossings  to  be 
discussed  later. 

Only  minor  modifications  result  from  a  choice  of  the  individual 
crossing,  whether  "  Peggs  point,"  "  New  Hamburg,"  or  "  Danskam- 
mer."  In  one  of  them,  New  Hamburg,  it  would  appear  possible  to 
cross  the  actual  river  section  wholly  in  slates.  This  seems  to  be  the 
reasonable  conclusion  from  the  diamond  drill  boring  at  Cedar  Cliff. 
But  even  that  line  necessitates  crossing  at  least  two  fault  contact 
lines  immediately  at  the  east  bank  and  beneath  Wappinger  creek  at 
depths  not  immensely  less  than  that  below  the  river  itself  and  both 
wholly  within  the  range  of  influence  of  the  river  waters.  It  would 
appear  therefore  that  the  situation  is  not  materially  altered  in  the 
present  discussion,  no  matter  which  particular  crossing  of  this 
group  is  considered. 

2  The  Highlands  group  [see  cross  section].  In  this  group  of 
crossings  there  are  two  separate  features  to  consider,  (a)  the 
Moodna  creek  valley  which  these  lines  all  cross,  and  (b)  the  Hud- 
son river  itself.    Their  characteristics  are  as  follows : 

a  Moodna  creek  [sec  separate  Moodna  creek  discussion].  So 
far  as  known  Moodna  creek  can  be  crossed  almost  wholly  in  slate. 
It  is  possible  that  the  underlying  limestone  may  come  near  enough 
to  the  rock  floor  of  the  valley  to  be  penetrated  but  there  is  little 


104 


NEW  YORK  STATE  MUSEUM 


direct  evidence  of  it.  The  ancient  valley  is  deep  and  probably 
marks  a  line  of  displacements  which  can  not  be  avoided,  no  matter 
what  route  is  chosen.  The  fault  contact  at  the  border  of  the  High- 
lands is  not  expected  to  prove  troublesome  as  it  seems  very  tight 
at  the  exposures  seen.  The  buried  granite  ridge  (a  continuation  of 
Snake  hill)  which  underlies  the  western  end  is  now  known  to  come 
within  the  limits  of  the  tunnel  and  adds  one  more  complication. 

Except  for  the  fact  that  the  ancient  Moodna  valley  is  deep  and 
filled  with  heavy  drift  that  is  unusually  difficult  to  prospect,  there 
would  seem  to  be  no  source  of  special  trouble.  It  has  no  lines  of 
weakness  that  are  not  also  present  in  the  more  northerly  districts 
and  the  tunnel  has  chances  of  crossing  them  under  more  advan- 
tageous conditions  without  so  much  complication  with  the  lime- 
stone series  as  characterizes  the  New  Hamburg  group. 

b  Hudson  river.  Among  the  Highlands  group  of  crossings  there 
is  considerable  difference  of  structure  dependent  upon  the  exact 
location  of  the  crossing.  The  conditions  that  prevail  may  be  sum- 
marized as  follows : 

(1)  Storm  King  location.  This  is  wholly  in  massive  and  gneissoid 
granite.  The  rock  is  the  most  massive  and  substantial  body  of 
uniform  type  found  in  the  Highlands.  The  course  of  the  river 
indicates  some  weakness  in  that  direction.  This  weakness  may  be 
some  minor  crushed  zone  or  even  the  jointing  alone  that  prevails 
throughout  the  exposed  cliffs.  But  there  is  no  direct  evidence  of 
faulting,  cutting  the  line  and  such  crushing  as  may  be  encountered  is 
believed  to  have  originated  at  such  depth  and  under  such  conditions 
as  to  cause  no  large  disturbance.  The  freedom  of  this  formation 
from  all  bedding  structures  and  natural  courses  of  underground 
water  circulation  on  a  large  scale  is  an  additional  factor.  There  is 
absolutely  no  other  place,  within  the  region,  where  the  Hudson  river 
can  be  crossed  from  grade  to  grade  in  good  ground  of  a  single  type 
with  so  great  probability  of  avoiding  all  large  lines  of  displacement. 

(2)  Little  Stony  point  location.  The  conditions  that  prevail  at 
this  point  are  similar  to  those  that  characterize  the  Storm  King  line. 
The  only  known  difference  is  in  the  considerably  more  shattered 
condition  of  the  granite,  especially  on  the  west  shore  at  Crows  Nest. 
It  is  estimated  that  this  crossing  is  less  favorable  by  reason  of  just 
this  poorer  condition  of  the  rock  and  the  somewhat  greater  yielding 
to  regional  disturbances  that  it  seems  to  indicate. 

(3)  Arden  point  or  West  Point  location.  On  this  line  the  river 
would  be  crossed  in  the  gneiss  series  proper  instead  of  in  granite. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  105 

It  is  largely  an  ancient  stratified  series  much  metamorphosed  con- 
taining belts  of  interbedded  limestones,  quartzites,  and  schists,  in 
addition  to  the  more  substantial  feldspathic  gneiss.  The  eastern 
bank  of  the  river  bears  also  abundant  evidence  of  extensive  crush- 
ing and  shearing  and  is  believed  to  indicate  a  displacement  in  this 
zone.  For  these  reasons  the  West  Point  crossing  is  considered  an 
unfavorable  route  compared  to  either  of  the  others  of  the  High- 
lands group. 

Summary.  In  a  comparison  of  the  geologic  features  that  are 
of  most  importance  in  contrasting  the  possible  routes  for  the  Hud- 
son river  crossing  the  following  points  are  considered  of  most 
importance. 

1  The  New  Hamburg  group  of  crossings  involves  (a)  the  long- 
est tunnel,  (b)  the  more  complicated  structures,  (c)  the  greatest 
number  of  known  faults,  crush  zones,  and  related  disturbances, 
(d)  the  more  variable  series  of  rock  types  to  be  penetrated,  (e) 
the  greater  tendency  to  encounter  heavy  underground  water  circula- 
tion, (/)  the  greater  probable  susceptibility  to  disturbance  from 
future  earth  movements,  and  (g)  the  greater  number  of  uncer- 
tainties of  rock  relations. 

2  In  contrast  the  Highlands  group  admits  of  (a)  shorter  total 
tunnel  length,  (b)  the  most  profound  fault  lines  of  the  district  are 
crossed  either  in  high  ground  or  are  avoided  or,  because  of  the 
rocks  involved,  promise  the  least  possible  trouble,  (c)  the  Hudson 
river  itself  can  be  crossed  in  a  single  formation  with  probability  of 
avoiding  lines  of  largest  structural  weakness  confining  the  greatest 
pressures  and  deepest  tunnel  work  within  the  most  uniform  and 
substantial  rock  of  the  whole  region. 

There  are,  of  course,  many  unknown  or  only  partially  known 
features  obscured  beneath  the  covering  of  drift  or  lying  beneath 
the  river  itself ;  but,  however  many  there  may  be,  it  is  not  believed 
that  they  can  materially  change  the  general  situation.  The  major 
characteristics  are  so  well  marked  that  any  addition  to  those  already 
known  would  in  all  probability  increase  the  difficulties  of  the  New 
Hamburg  group  of  routes  at  least  as  much  as  and  perhaps  more 
than  those  of  the  Highlands  group. 

In  view  of  the  above  facts  and  inferences  the  judgment  has  been 
in  favor  of  the  Highlands  group  of  crossings  as  the  more  defensible 
on  geologic  grounds  as  a  route  for  the  aqueduct  line.  Furthermore, 
in  accord  with  the  preferences  already  noted,  the  Storm  King  loca- 
tion is  regarded  as  the  most  likely  to  give  satisfactory  results. 


ioC 


NEW  YORK  STATE  MUSEUM 


Quality  and  condition  of  rock 

The  rock  of  Storm  King  mountain  and  of  Breakneck  ridge  at  the 
Hudson  river  crossing  is  a  very  hard  granite  with  a  gneissoid 
structure  of  variable  prominence.  The  color  varies  from  grayish 
to  light  reddish  and  the  structure  is  always  coarse  passing  into 
pegmatite  facies  that  occur  as  stringers  or  irregular  veinlets.  The 
grayish  facies  is  of  slightly  finer  grain  and  more  gneissoid.  Those 
portions  that  have  been  sheared  are  still  darker.  There  are  many 
joints  at  the  surface  running  at  various  angles  and  an  occasional 
slickensided  surface.  The  mass  is  cut  by  several  dikes  of  more 
basic  rock  (diorite)  of  widths  varying  from  a  few  inches  to  8  feet. 
These  dikes  are  somewhat  more  closely  jointed  than  the  granite 
and  consequently  a  little  more  readily  attacked  by  the  weather.  But 
where  protected  they  are  equally  substantial  for  underground  work. 

The  chief  variation  from  ibis  condition  is  where  crushing  or 
shearing  has  induced  metamorphic  changes.  Wherever  bed  rock 
has  been  reached  at  this  point  and  to  such  depths  as  workings  have 
penetrated  the  rock  is  of  this  type. 

The  work  includes  (a)  four  inclined  drill  holes  from  the  river 
margin  —  two  starting  from  the  surface  and  two  from  chambers 
set  off  from  shafts  at  a  starting  depth  of  about  200  feet,  (b)  several 
vertical  holes  in  the  river  itself,  and  (c)  two  large  working  shafts 
20  x  20,  one  on  either  side  of  the  river. 

These  give  all  the  data1  known  as  to  the  condition  at  depth.  From 
them  it  is  apparent  that  crushing  and  shearing  have  been  prominent. 
Many  splendid  specimens  of  crush  breccia  are  thrown  on  the  shaft 
dump.  But  its  present  condition  at  the  depth  involved  is  sound  and 
durable.  The  fractures  are  rehealed.  There  has  been  a  recombina- 
tion of  constituents  giving  a  new  matrix  of  complex  silicates  among 
which  epidote  is  the  most  characteristic,  while  simple  decay  is  of 
little  consequence.  For  strength  and  permanence  the  conditions 
could  not  well  be  improved.  There  is  no  reason  to  apprehend  any 
change  for  the  worse  for  the  reason  that  the  same  tendencies  must 
prevail  at  that  depth  throughout.  It  would  appear  therefore  that 
faulting  movements,  or  the  existence  of  a  fault  zone  of  importance 
can  not  become  a  serious  obstruction,  because  of  the  tendency  to 


1  Since  this  paragraph  was  written  four  inclined  diamond  drill  borings 
have  .been  made  from  chambers  at  depths  of  about  200  feet  in  the  shafts. 
These  have  now  penetrated  the  whole  distance  beneath  the  Hudson  with 
very  satisfactory  results. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  107 

lieal  up  the  fractures  and  so  make  the  rock  about  as  substantial  as 
before. 

It  is  noted  elsewhere  that  faulting  is  common  in  this  region,  and 
that  in  a  considerable  portion  of  its  lower  course  the  Hudson  prob- 
ably follows  such  structures.  It  is,  however,  wholly  unnecessary 
to  assume  that  its  whole  course  is  a  fault  line.  WJiether  or  not 
there  is  a  longitudinal  fault  zone  of  any  prominence  in  the  river  at 
Storm  King  is  unknown.  There  are  several  cross  faults,  both  above 
and  below  this  point,  that  give  much  clearer  surface  evidence  of 
their  presence.  Fault  zones  have  proven  to  be  objectionable  ground 
in  many  places  along  the  aqueduct  line,  but  elsewhere  the  data 
refer  chiefly  to  situations  favoring  more  ready  underground  cir- 
culation, i.  e.  at  higher  levels.  In  this  particular  case  the  rock  in 
question  lies  below  former  ground  water  level  within  the  belt  of 
cementation  rather  than  up  in  the  belt  of  decay,  and  there  is  prob- 
ably no  disintegrated  rock  from  any  cause. 


CHAPTER  IV 


GEOLOGICAL   FEATURES   INVOLVED    IN    SELECTION  OF 
SITE  FOR  THE  ASHOKAN  DAM 

Topographic  features  of  the  southeastern  margin  of  the  Cats- 
kills,  where  the  chief  water  supply  is  available,  fixes  the  approxi- 
mate location  and  bounds  of  the  principal  reservoir.  The  accom- 
panying map,  a  portion  of  the  western  part  of  Rosendale  quad- 
rangle, shows  the  situation.  The  part  of  the  work  involving  the 
chief  geological  problem  was  the  choice  of  the  principal  dam  sites 
on  the  Esopus.  This  is  known  as  the  Ashokan  dam.  This  part  of 
the  Catskill  system  belongs  to  the  Reservoir  Department  under 
Mr  Carlton  E.  Davis  as  department  engineer. 

There  were  originally  considered  three  sites:  (i)  at  "  Broadhead 
bridge,"  (2)  at  "  Olive  Bridge,"  (3)  at  "  Cathedral  gorge"  or  the 
"  Tongore  "  site.  Any  one  of  these  seemed  possible  from  a  topo- 
graphic standpoint.  Later  developments  in  regard  to  storage  ca- 
pacity and  engineering  considerations  finally  reduced  the  practicable 
sites  to  two  —  the  ''Olive  Bridge"  and  the  "Tongore."  These 
were  then  explored  thoroughly  as  an  aid  to  determining  whether 
or  not  there  were  favorable  or  unfavorable  conditions  at  either 
location.  Trenches  were  dug,  shafts  were  sunk,  wash  holes  were 
put  down,  and  drill  borings  were  made.  The  amount  of  such  work 
done  was  sufficient  to  show  the  actual  conditions  both  of  the  drift 
and  bed  rock  and  incidentally  to  throw  some  light  on  minor  matters 
in  geologic  history. 

This  discussion  is  essentially  a  summary  of  these  data  and  a  com- 
parison of  the  geologic  conditions  indicated  by  the  explorations1  of 
these  two  sites  and  a  statement  of  some  of  the  geologic  character- 
istics of  the  area. 

1  General  geologic  conditions  as  shown  by  the  explorations 

Bed  rock  is  dark  colored  Devonic  sandstone  and  shale,  the 
Sherburne  formation,  lying  almost  horizontal,  strongly  jointed, 
plainly  bedded,  and  of  good  quality  for  the  foundation  of  the  dam. 

At  both  locations  the  present  Esopus  flows  in  a  postglacial  gorge 

1  In  this  work  of  exploration  a  very  efficient  staff  of  engineers  was  engaged. 
Among  those  having  very  much  to  do  with  the  features  here  discussed  are 
Thaddeus  Merriman,  division  engineer,  J.  S.  Langthorn,  division  engineer 
and  Sidney  Clapp,  assistant  engineer. 

[109] 


no 


NEW  YORK  STATE  MUSEUM 


and  there  is  a  somewhat  deeper  huried  channel  a  short  distance  to 
the  north  side.  In  each  case  this  old  channel  bed  rock  is  probably 
less  fresh  and  substantial,  due  to  former  weathering,  than  the  pres- 
ent exposed  surfaces. 

In  each  case  glacial  deposits  reach  a  thickness  of  more  than  200 
feet  within  the  narrow  valley  or  gorge,  especially  along  the  north 
valley  wall  within  the  limits  of  the  proposed  dam. 

Special  geological  conditions.  The  factors  in  which  there  is 
most  variation  and  which  are  of  most  significance  in  a  comparative 
study  are  those  belonging  to  the  glacial  drift  deposits.  In  order  to 
properly  estimate  the  influence  of  some  of  these  features  it  will 
be  necessary  to  briefly  consider  the  types  of  material  represented 
at  different  places  and  the  conditions  under  which  they  were 
formed. 

Types  of  material.  Till.  Heavy  bouldery  till,  mixed  clay, 
sand,  gravel,  and  boulders,  is  the  most  abundant  type  of  material. 
It  forms  especially  the  chief  surface  material  throughout  the  region, 
and  is  the  surface  type  at  both  sites.  It  becomes  at  places  quite 
sandy,  but  is  almost  everywhere  good,  impervious  material  because 
of  its  mixed  character. 

Laminated  till.  At  a  few  places,  notably  in  the  Beaver  kill  near 
its  mouth,  and  in  a  trench  above  Olive  Bridge,  and  in  the  "  big 
ciugway  "  above  West  Shokan,  strong  lamination  appears  in  heavy 
stony  till  as  if  laid  down  rapidly  in  comparatively  quiet  water  such 
as  the  margin  of  a  lake.   This  material  is  especially  impervious. 

Gravel  hillocks.  A  few  small  hillocks  with  morainic  contour, 
indicating  a  dumping  ground  for  some  glacier  on  a  small  scale, 
occupy  the  flat  immediately  west  of  Browns  Station.  They  were, 
at  a  very  late  stage,  piled  into  the  course  of  a  former  glacial  stream 
whose  delta  deposits  occupy  the  sandy  bench  above  the  500  foot 
contour  just  north  of  Olive  Bridge. 

Assorted  gravel  and  sand.  This  material  is  abundantly  developed 
just  north  of  Olive  Bridge.  It  seems  to  have  formed  a  delta  de- 
posit at  the  mouth  of  a  glacial  stream  that  emptied  into  the  main 
valley  at  this  point.  The  running  water  washed  almost  all  of  the 
clay  and  extremely  fine  material  farther  out,  where  they  settled  in 
the  bottom  of  a  small  glacial  lake  that  was  at  that  time  held  in  this 
upper  portion  of  the  Esopus  valley.  The  dam  that  held  in  this  body 
of  water  which  reached  above  the  520  foot  line  stood  near  the 
proposed  "  Olive  Bridge  "  dam  site.  The  materials  forming  the 
dam  were  in  part  the  glacial  till  that  is  now  found  on  that  site  and 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


III 


in  part  the  ice  itself,  which  came  in  from  the  northeast,  helping  to 
complete  the  barrier.  Into  this  lake  the  streams  from  the  melting 
ice  margins  deposited  their  load  of  silt.  This  is  well  shown  in  the 
trenches  cut  across  the  terrace  }i  of  a  mile  above  Olive  Bridge. 

A  similar  occurrence  is  seen  at  the  cemetery  near  West  Shokan. 

Laminated  sand  and  clay.  In  all  cases  where  silts  were  carried 
into  the  lake  basin  the  finest  materials  were  carried  in  suspension 
to  greater  distances  from  the  margins,  and  slowly  settled  out  in  the 
form  of  alternate  laminae  of  clay  and  fine  sand.  Each  sandy  layer 
represents  a  fresh  supply  of  material  and  rapid  precipitation  of  the 
comparatively  heavy  grains ;  while  each  clay  layer  represents  a 
period  of  greater  quiet  or  decreased  supply  during  which  the  finest 
particles  settled  to  the  bottom.  A  predominance  of  fine  sand  indi- 
cates either  abnormal  supply  or  proximity  to  the  supply  margin, 
while  a  predominance  of  clay  represents  either  uniform  and  mod- 
erate supply  or  greater  distance  from  the  supply  margin. 

These  deposits  are  nearly  impervious  to  water  moving  vertically, 
but  much  more  pervious  laterally  and  especially  so  in  the  most 
sandy  portions  forming  the  marginal  facies. 

This  type  of  deposit  is  to  be  seen  at  the  surface  at  about  the 
700  foot  contour  2  miles  north  of  Shokan,  above  the  "  big  dugway," 
also  in  the  trenches  cut  into  the  terrace  at  about  the  500  foot  con- 
tours Y\  of  a  mile  north  of  Olive  Bridge,  and  it  is  probable  that 
this  same  type  underlies  the  northern  half  of  the  "  Tongore  "  site. 
The  material  marked  "  fine  sand  "  at  and  below  the  400  foot  line 
on  the  accompanying  "  geologic  section  "  G-H  is  judged  to  be  of 
this  type. 

Pebbly  clays.  These  are  developed  to  only  limited  extent  and 
indicate  probably  floating  ice  in  addition  to  the  other  methods  of 
distribution. 

Gravel  streaks  and  assorted  pebble  beds.  Wherever  water  flowed 
with  considerable  current  across  the  material  either  before  or  after 
deposition  the  finer  particles  were  removed  and  only  gravel  and 
pebbles,  too  heavy  to  transport,  were  left  behind.  Some  of  these 
gravel  beds  were  developed  in  the  intervals  of  successive  advances 
and  retreat  of  the  ice  when  for  a  time  the  lower  valleys  were  unoc- 
cupied. In  many  places  the  succeeding  advance  of  the  ice  would 
plow  all  these  surface  materials  up  again  and  mix  them  into  the 
usual  till ;  but  occasionally  the  oncoming  glacier  simply  covered 
these  deposits  with  its  own  till  mantle,  and  they  are  preserved  as 
records  of  these  minor  interglacial  stages.    Such  behavior  would  be 


112 


NEW  YORK  STATE  MUSEUM 


more  likely  to  occur  in  the  deeper  channels.  To  this  class  of 
deposit  belong  some  of  the  gravel  beds  of  the  "  Tongore  "  site, 
notably  that  shown  in  one  of  the  deep  shafts.  It  is  probable  that 
the  zones  where  the  wash  rig  experienced  a  "  loss  of  water  "  are 
most  of  them  of  this  type. 

I 

2  Summary  of  geologic  history 

In  preglacial  time  the  Esopus  valley  was  occupied  by  a  stream 
of  similar  capacity  to  the  present  Esopus  creek.  Its  channel  lay 
to  the  north  side  of  the  narrow  valley,  having  adjusted  itself  in 
conformity  to  the  slight  dips  of  the  Hamilton  sandstones  and  its 
principal  joints.  At  the  points  under  investigation  this  original 
channel  is  buried  under  several  kinds  of  glacial  deposits  whose 
source  of  accumulation  was  chiefly  from  the  north  and  northeast, 
blocking  the  stream  channel  and  forcing  the  stream  to  the  opposite 
(south)  side.  The  direction  of  movement  was  favorable  to  the  dam- 
ming of  the  Esopus  creek  valley  and  the  deposits  indicate  that 
this  occurred  at  several  different  times  and  at  different  elevations 
and  that  corresponding  lake  conditions  occasionally  prevailed.  It 
is  equally  clear  that  there  were  intervals  of  retreat  of  the  ice  with 
attendant  stream  action  and  the  development  of  gravel  beds,  fol- 
lowed by  another  ice  advance,  either  obliterating  the  surface 
features  or  covering  the  previous  deposits  with  another  till  layer. 
With  each  successive  withdrawal  the  local  streams  found  them- 
selves more  or  less  completely  out  of  place,  and  consequently  their 
characteristic  deposits  formed  in  these  intervals  may  be  found  in 
unlooked  for  places  wholly  inconsistent  with  present  surface 
contour. 

At  the  final  withdrawal  of  the  ice,  Esopus  creek  found  itself 
entrenched  along  the  southern  margin  of  the  valley  and  has  cut  a 
postglacial  rock  gorge  instead  of  removing  the  compact  till  from  the 
original  channel.  But  wherever  only  modified  drift,  either  sand  or 
clay,  was  the  valley  filling  it  scooped  out  great  bends  so  that  a  large 
proportion  of  this  type  has  been  removed  from  the  valley,  and 
only  the  margins  remain  as  terraces  or  covered  beneath  other  pro- 
tecting deposits. 

" "  .  i 

3  Application  to  the  choice  of  dam  site 

o  "  Olive  Bridge "  site.  The  trenches  and  shafts  together 
with  surface  exposures  indicate  that  the  glacial  drift  at  the  Olive 


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GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


"3 


Bridge  site,  at  one  stage  in  the  glacial  history,  served  as  a  natural 
dam  and  that  water  was  successfully  held  above  it  to  an  elevation 
of  530  feet  and  perhaps  more. 


CiTyOFNcwyvnit  - 
BOARD  OF  WATER  SUPPLV 
ASHOKAN  RESERVOIR 
OLIVE  BRIDGE  QAM  SITE 
....     _    _     ..  — 


SmcltO"    of  Sif«  On  ConttrLm* 


On       Act  At 


Fig  16  Location  of  the  Ashokan  dam  at  Olive  Bridge  site  and  a  geologic 
cross  section.  The  small  dots  in  the  plan  indicate  exploratory  borings.  The 
section  shows  the  rock  profile  indicating  a  preglacial  channel  of  the  Esopus. 
The  present  Esopus  flows  in  a  new  postglacial  ^channel  at  a  higher  elevation. 


The  lowest  materials  in  contact  with  bed  rock  are  heavy  stony 
till,  laminated  till  and  stony  laminated  clays  —  all  good  impervious 
material  wherever  exposed  and  tight  upon  bed  rock.  Sands  and 
laminated  clays  are  extensively  developed  immediately  northward 
of  the  site  and  streaks  of  these  deposits  interlock  to  a  limited  extent 
with  the  till  materials  of  the  site  itself,  but  they  do  not  extend  far 
and  die  out  in  wedges  among  the  heavy  deposits  that  characterize 
the  southern  slopes  of  the  hill  forming  the  northern  terminus  of 
the  dam.  These  pervious  streaks  do  not  extend  at  any  point  con- 
tinuously through  this  hill  and  consequently  as  a  whole  the  present 
barrier  as  it  stands  is  practically  impervious.  The  poorer  materials 
(assorted  gravels  and  sands)  characterize  the  upstream  side,  and 
the  better,  more  impervious  materials  (till  and  laminated  boulder 
■clays)   characterize  the  downstream  side  of  the  proposed  Olive 


NEW  YORK  STATE  MUSEUM 


Bridge  site.  It  is  therefore  advisable  to  locate  any  such  structure 
as  a  dam  at  a  point  as  far  down  stream  on  this  site  as  other  engi- 
neering factors  permit. 

b  "  Tongore  "  site.  At  Tongore,  bed  rock  is  at  least  a  hun- 
dred feet  deeper  than  at  Olive  Bridge.  In  the  deeper  parts,  below 
the  400  foot  line  the  deposits  as  indicated  by  the  wash  borings  [see 
sections]  are  interpreted  as  a  fairly  continuous  succession  of  till, 
stratified  sands  and  gravels,  and  laminated  sands  and  clays  belong- 
ing to  two  or  three  different  stages  of  accumulation.  Upon  tins 
the  heavy  upper  till  was  laid  down.  It  is  believed  that  the  records 
fully  support  this  view  and  that  the  stratified  or  laminated  materials 
were  accumulated  at  a  time  when  a  temporary  dam  existed  at  some 
point  still  farther  down  the  Esopus  valley.  It  is  apparent  further- 
more that  the  most  porous  zone  is  at  the  junction  of  the  upper 
till  and  the  lower  stratified  deposits  and  in  part  is  represented  by 
the  assorted  pebbles  of  stream  wash  —  in  general  not  far  from  the 
400  foot  line.  These  middle  zone  deposits  are  believed  to  extend 
continuously  through  the  drift  ridge  that  forms  the  northern  half 
of  this  site.    As  before  noted,  though  rather  impervious  vertically, 


Fig.  17  Plan  and  geologic  section  at  the  Tongore  site.  The  dots  on  the  map  indicate 
exploratory  borings  and  the  course  of  the  buried  channel  of  the  preglacial  Esopus  creek 
is  shown  making  a  right  angle  bend  to  the  north.  The  section  shows  the  buried  chan- 
nel, the  new  postglacial  channel  and  the  great  accumulation  of  porous  modified  drift 
which  is  regarded  as  one  important  objection  to  this  site  for  the  dam. 

some  of  these  deposits  allow  ready  lateral  movement  of  water.  This 
is  held  to  account  for  the  rather  persistent  occurrence  of  springs 
or  seepage  along  the  creek  bank  at  about  this  level  both  above  and 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


Il6  NEW  YORK  STATE  MUSEUM 

below  the  site.  The  great  thickness  of  these  laminated  beds,  in 
places  a  hundred  feet  or  more,  together  with  the  abundance  of  sand 
in  them,  and  the  caving  tendencies  exhibited  by  them  in  one  of  the 
large  shafts,  indicates  poor  conditions  for  such  a  piece  of  work. 

The  behavior  of  one  of  the  test  shafts  throws  some  light  on  con- 
ditions within  the  drift  deposits.  At  this  place  after  sinking  into 
the  underlying  gravel  beds  there  was  "  no  water  "  at  first,  but  after 
going  a  few  feet  deeper  there  was  an  abundant  flow,  that  did  not 
rise  much  in  the  shaft.  This  case  seems  to  support  the  following 
interpretation. 

The  gravels  encountered  do  not  form  an  isolated  pocket  or  lens, 
else  it  would  have  carried  water  full  from  the  first.  It  must  be 
a  fairly  continuous  porous  zone  with  large  feeding  connections  else 
it  would  run  dry,  and  it  must  have  an  easy  discharge  else  it  would 
have  risen  above  the  level  of  the  first  gravels.  Therefore  it  must 
be  a  rather  well  marked  subterranean  water  passage  or  porous  zone 
of  considerable  extent.  Such  conditions  would  make  an  impervious 
core  wall  to  bed  rock  at  this  site  a  necessity  and  its  construction  a 
matter  of  considerable  difficulty.  At  this  site  also  because  of  the 
small  cross  section  of  the  ridge,  there  is  little  chance  for  the  inter- 
locking of  layers  or  the  blocking  of  the  porous  ones  by  a  till  barrier 
to  check  the  lateral  seepage,  and  there  is  no  chance  to  move  farther 
down  stream  to  secure  such  conditions. 

4  Summary 

Because  of  the  (a)  higher  bed  rock  throughout,  and  (b)  the 
more  uniform  and  impervious  quality  of  drift  deposits,  and  ( c) 
the  more  massive  cross  section  of  drift  barrier  for  foundation,  and 
(d)  the  perfectly  tight  contacts  of  till  and  bed  rock,  and  (e)  the 
limitation  of  the  more  porous  materials  to  higher  levels  and  (/)  the 
glacial  history  connected  with  the  development  of  all  these  parts. 
"  Olive  Bridge  "  is  the  preferable  location  for  the  proposed  Asho- 
kan  dam  on  Esopus  creek. 


CHAPTER  V 


CHARACTER    AND    QUALITY    OF    THE    BLUESTONE  FOR 
STRUCTURAL  PURPOSES 

Probably  no  stone  marketed  in  New  York  State  is  more  exten- 
sively known  than  the  "  bluestone  "  of  the  Catskill  region.  But  it 
is  noted  particularly  for  a  special  purpose,  i.  e.  as  flagstone,  because 
of  its  capacity  to  part  or  cleave  into  thin  slabs.  These  slabs  are 
proven  by  experience  to  have  remarkable  weather  resistance  and 
durability. 

Little  attention  has  been  given  to  the  question  of  dimension  stone 
—  whether  or  not  such  blocks  of  as  high  quality  as  the  flags  could 
be  obtained  and  where  such  quarries  could  be  opened. 

There  are  several  reasons  for  this  situation.  In  the  first  place 
(i)  the  stone  is  of  a  dark  color  and  has  a  dull  appearance  so  that 
it  is  not  fancied  for  the  usual  expensive  structures  where  large  sizes 
are  used,  also  (2)  the  quarries  are  small,  shallow,  and  are  worked 
on  a  small  scale  by  single  individuals  or  groups  of  neighbors  with 
few  quarrying  tools  and  no  transportation  facilities  for  large  mate- 
rial, and  in  addition  (3)  considering  the  work  and  equipment  neces- 
sary and  the  demand  the  flag  industry  was  more  profitable. 

Because  of  the  large  demands  of  the  Ashokan  dam  where  nearly 
a  million  cubic  yards  of  heavy  masonry  construction  are  to  be  used 
an  entirely  new  situation  has  developed.  It  is  especially  desirable 
that  a  rock  capable  of  furnishing  heavy  dimension  blocks  should 
be  discovered.  The  usual  slab  or  flag  type  is  unsuited  to  a  consider- 
able part  of  this  work.  A  study  of  the  adjacent  region  therefore 
has  been  made  and  explorations  along  certain  promising  lines  have 
been  conducted  to  sufficient  completeness  to  prove  that  a  suitable 
stone  can  be  furnished  in  large  quantity.  The  characteristics  of 
structure  and  occurrence  as  shown  by  this  special  study  are  given, 
together  with  some  of  the  later  exploratory  data. 

Physiographic  features1 

All  of  the  rock  formations  are  sedimentary,  chiefly  sandstones 
and  shales.  They  lie  in  alternating  beds  of  variable  thickness  and 
are  almost  horizontal.   The  total  thickness  is  many  hundred  feet  so 

1  The  principal  argument  of  this  discussion  has  been  used  in  a  previous 
article  by  the  writer  under  the  title  "  Quality  of  Bluestone  in  the  vicinity 
of  the  Ashokan  Dam"  in  the  School  of  Mines  Quarterly,  v.  20,  no.  2. 

"7 


n8 


NEW  YORK  STATE  MUSEUM 


that  neither  the  hottom  nor  the  top  beds  of  the  series  are  to  be  seen 
in  this  locality. 

The  region  is  one  of  considerable  relief  representing  preglacial 
erosion.  The  glacial  drift  mantle  has  modified  it  chiefly  by  obscur- 
ing some  of  the  smaller  irregularities  of  rock  contour,  and  espe- 
cially by  partially  filling  many  of  the  stream  gorges.  Postglacial 
erosion  has  not  completely  reexcavated  the  old  channels.  But  the 
contour  of  the  uplands  reflects  the  character  of  the  bed  rock  with 
considerable  success.  The  tendency  of  the  more  massive  and  coarse 
grained  varieties  of  rock  to  resist  weathering  and  erosion  more  suc- 
cessfully than  the  finer  grained  and  more  argillaceous  or  shaly 
facies  is  a  general  characteristic.  Since  these  varieties  form  suc- 
cessive or  alternating  beds  throughout  the  whole  area,  the  result  is 
an  almost  universal  cliff -and-slope  surface  form.  This  bed  rock 
topography  is  somewhat  obscured  but  not  wholly  obliterated  by 
glacial  erosion  and  deposition.  Therefore  it  may  be  used  with  con- 
fidence in  locating  or  tracing  the  more  durable  beds  since  they 
almost  invariably  appear  as  a  shelf  or  terrace  with  a  steep  margin 
toward  the  lower  side  and  a  gentle  slope  on  the  rising  side. 

Structural  features 

The  rock  types  include  bluish  gray  or  greenish  gray  sandstones 
with  almost  horizontal  bedding,  and  sometimes  exhibiting  cross- 
bedding  structure,  and  compact  very  dark  argillaceous  shales.  These 
two  are  of  about  equal  prominence,  but  only  the  sandstone  is  of 
importance  in  the  present  discussion.  Tts  minute  structure  will  be 
given  in  greater  detail  in  the  petrographic  discussion. 

Jointing  is  common  and  persists  in  two  sets  nearly  at  right  angles 
to  each  other  —  one  striking  northeastward  and  the  other  toward 
the  northwest.  In  some  of  the  best  exposures,  these  joints  are 
clear-cut  and  run  10  to  18  feet  apart,  dipping  almost  vertically.  In 
the  more  massive  beds  there  is  very  little  small  jointing,  so  that  the 
character  is  especially  favorable  to  large  dimension  work. 

But  still  more  prominent  structures  are  the  partings  which  follow 
the  bedding  planes.  These  give  the  rock  a  decided  tendency  to 
cleave  naturally  into  slabs,  the  uppermost  exposed  portion  of  almost 
every  outcrop  exhibiting  this  slab  structure  in  more  or  less  perfec- 
tion. So  general  is  this  structure  at  all  horizons  in  the  sandstones 
of  the  series  that  there  can  be  no  doubt  of  its  connection  with 
some  original  sedimentation  character.  Besides  it  is  a  potential 
factor  in  nearly  all  the  beds  even  when  not  very  apparent.  The 


GEOLOGY  Of  THE  NEW  YOftK  CITY  AQUEDUCT 


itg 


exposed  places  exhibit  the  character  so  prominently  only  because 
of  the  weathering  effect,  which  develops  the  natural  tendency.  This 
general  conclusion  is  borne  out  by  the  well  known  practice  of  quar- 
rymen  of  the  district  of  splitting  the  larger  blocks  into  slabs  of  the 
required  thickness  by  wedges  driven  along  certain  streaks  that  are 
known  as  "  reeds."  A  reeding  quarry  is  one  that  has  this  capacity 
well  developed,  and  it  is  this  character  in  part  that  has  made  the 
"  bluestone  "  or  "  flagstone  "  of  New  York  an  important  factor  in 
the  production  of  the  United  States  for  a  great  many  years. 

For  large  size  dimension  stone  where  great  stress  is  involved  it 
is  evident  that  this  structure  would  not  be  desirable.  These  definite 
planes  of  weakness  reduce  the  general  efficiency.  A  little  observa- 
ti  jn  however  shows  that  there  are  some  outcrops  and  an  occasional 
quarry  where  the  more  massive  blocks  do  not  split  well.  From  the 
necessities  of  the  industry  these  have  been  avoided  or  but  meagerly 
developed.  In  some  cases  of  this  kind  the  sedimentation  is  of  the 
cross-bedded  type  with  somewhat  interlocked  laminae.  If  the  grain 
is  coarse  such  varieties  resist  splitting  with  great  success.  The 
thickness  of  such  beds  varies  from  a  few  feet  to  25  feet  or  even 
more  without  prominent  interbedding  of  shale  layers. 

Stratigraphy.  These  are  the  sandstones,  flags  and  shales 
known  as  the  Hamilton,  Sherburne  and  Oneonta  formations  belong- 
ing to  the  Devonic  period.  The  strata  of  the  immediate  vicinity  of 
this  examination  belong  to  the  Sherburne  subdivision,  but  no  at- 
tempt to  differentiate  the  formations  was  made.  Structurally  and 
petrographically  the  different  formations  are  not  distinguishable  in 
this  area.  On  the  market  the  stone  from  either  is  known  generally 
as  "  Hamilton  flag  "  or  "  Milestone." 

Economic  features 

There  are  hundreds  of  quarries  in  this  general  region.  Nearly  all 
are  small,  and  are  worked  on  a  small  scale  without  machinery.  The 
product  is  almost  wholly  thin  slabs  of  the  flagstone  type.  This  is 
supplemented  by  a  small  amount  of  somewhat  more  massive  char- 
acter, dressed  for  window  sills ;  and  a  very  limited  output  is  of 
dimension  stone  of  larger  size.  The  general  lack  of  suitable  me- 
chanical devices  and  transportation  facilities  are  the  chief  reasons 
for  the  limited  output  of  the  last  named  grades. 

Petrography 

The  basis  of  this  discussion  is  a  microscopic  examination  of  sev- 
eral thin  sections  made  of  the  different  types  of  rock  from  the 


120 


NEW  YORK  STATE  MUSEUM 


quarries  whose  field  geologic  features  give  promise  of  encouraging 
results.  The  most  characteristic  variations  are  illustrated  in  the 
accompanying  photomicrographs,  plates  22,  23. 

Texture.  The  rock  is  granular,  the  individual  grains  varying 
from  minute  particles  in  the  finer  shale  layers  to  three  or  four 
tenths  of  a  millimeter  in  diameter  in  the  coarser  sandstone  [pi.  23, 
lower  figure].  The  grains  are  seldom  rounded.  Jagged  or  frayed 
or  elongate  forms  are  the  rule  [pi.  23,  upper  figure].  There  is  no 
marked  porosity.  When  the  rock  was  first  deposited  as  a  sediment 
it  probably  had  the  usual  large  interstitial  spaces  of  such  rock  type, 
but  in  this  case  some  subsequent  modification  —  an  incipient  meta- 
morphism  —  has  largely  obliterated  the  voids  by  the  introduction  or 
development  of  mineral  matter  of  secondary  origin. 

In  general  it  is  quite  apparent  that  the  average  grain  was  orig- 
inally more  rounded  than  its  present  representative. 

Mineralogy.  The  original  minerals  in  order  of  abundance 
were  the  feldspars,  quartz,  and  probably  hornblende,  biotite,  and  in 
much  smaller  amounts  others  of  little  apparent  consequence  in  the 
present  discussion. 

All  of  these  have  been  more  or  less  affected  by  subsequent 
changes.  Quartz  has  suffered  least  of  all,  the  chief  modification 
being  a  greater  angularity  of  form  and  an  occasional  interlocking 
tendency  caused  by  secondary  growth  [pi.  22,  lower  figure]. 

Both  orthoclase  and  plagioclase  feldspars  occur.  The  orthoclase 
grains,  which  originally  made  up  more  than  half  of  the  bulk  of  the 
coarser  types  of  rock,  have  been  in  places  profoundly  altered  [pi.  22, 
upper  figure].  In  many  cases  the  identification  of  this  mineral  de- 
pends upon  its  association  and  the  abundant  remnants  of  character- 
istic structure  and  its  normal  secondary  products.  In  the  least 
affected  grains  satisfactory  identification  is  not  difficult.  Even  in 
the  most  modified  representatives  there  is  some  preservation  of 
structure  indicating  size  of  grain  and  proving  the  essentially  gran- 
ular character  of  the  rock.  The  plagioclase,  although  not  abundant, 
is  more  readily  detected  than  the  orthoclase  because  it  has  been 
much  less  affected  by  the  secondary  changes. 

All  original  ferromagnesian  constituents  are  wholly  altered.  There 
were  some  such  constituents  in  the  rock,  as  is  plainly  shown  by  the 
secondary  products.  Hornblende  and  biotite  were  probably  both 
present. 

The  secondary  products,  derived  from  the  original  feldspars  and 
ferromagnesian  constituents,  include  sericite,  chlorite,  calcite  and 


Plate  22 


Photomicrograph  of  bluestone,  x  25  diameters.  The  clearer  grains  are 
quartz  and  indicate  the  approximate  size  of  other  original  constituents. 
In  this  case  the  alteration  of  the  feldspars  and  ferromagnesian  originals 
is  so  complete  that  their  products  form  an  indeterminable  complex  aggre- 
gate of  closely  interlocked  granules,  flakes,  and  fibers  of  extremely  fine 
texture. 


Photomicrograph  of  first  grade  medium  grain  bluestone,  x  25  diameters. 
Taken  to  show  angular  and  interlocking  grains  indicating  secondary 
growth  and  a  complete  lack  of  reeding  structure.  The  clear  grains  are 
quartz;  the  rest  of  the  field  is  made  up  chiefly  of  secondary  derivatives 
from  the  original  feldspars  and  ferromagnesian  minerals. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  121 

quartz  as  the  most  important  and  abundant.  Others  probably  occur 
that  are  less  readily  differentiated,  and  among  them  is  kaolin.  Occa- 
sionally a  small  amount  of  massive  or  granular  pyrite  occurs.  There 
are  traces  of  organic  remains,  especially  plant  stems,  and  the  pyrite 
is  most  plentiful  in  association  with  those  beds. 

It  seems  to  be  the  secondary  products  largely  that  give  the  char- 
acteristic bluish  or  greenish  color  to  this  stone.  Practically  all  of 
the  iron  freed  by  secondary  changes  from  the  ferromagnesian  con- 
stituents has  entered  into  new  silicate  compounds,  especially  with 
the  chlorite,  which  are  minutely  distributed  throughout  the  whole 
mass,  giving  it  all  a  tinge  of  the  characteristic  color  of  these  well 
known  products.  The  same  amount  of  iron  in  the  oxid  form 
would  no  doubt  give  as  highly  colored  stone  as  any  of  the  reds  or 
browns  of  other  familiar  types  of  sandstone.  But  the  tendency  to 
form  the  sericite-chlorite-quartz  aggregate  in  the  rock  has  also  an 
important  bearing  on  its  durability  and  strength.  This  is  further 
discussed  in  a  separate  paragraph. 

Classification.  It  is  clear  that  this  type  of  bluestone  is  a  sedi- 
mentary rock  of  medium  grain,  a  sand  rock  or  "  renyte."  Since 
the  silicates  are  so  predominant  in  the  original  composition  it  may 
be  further  identified  as  a  sandstone  or  a  "  silicarenyte."  But  in 
view  of  the  predominance  of  the  feldspars  it  should  be  further 
designated  as  an  arkose  sandstone.  And  considering  the  extent  to 
which  it  has  been  modified  by  the  development  of  interstitial  sili- 
cious  products  and  the  effect  that  this  has  had  in  perfecting  the 
bond  between  the  grains,  the  rock  may  be  classified  as  an  indurated 
arkose  sandstone. 

Special  structure.  A  study  of  the  cause  of  reeding,  or  the 
tendency  to  split  into  slabs,  led  to  the  preparation  of  thin  sections 
of  this  structure  [pi.  23,  upper  figure].  It  is  apparent  from  them 
that  the  reed  is  strictly  a  rock  structure  and  that  the  perfection  of 
the  capacity  to  split  along  these  planes  depends  wholly  upon  the 
abundance  and  arrangement  and  size  of  the  elongate  and  semifibrous 
grains  and  the  presence  of  a  more  than  usual  amount  of  original 
fine  or  flaky  material.  Almost  universally  the  reed  streaks  are 
darker  in  color  and  finer  in  grain  than  the  average  of  the  rest  of  the 
rock. 

In  part  therefore  it  is  an  original  character  due  to  the  assorting 
action  of  water  during  deposition,  finer  streaks  alternating  with 
coarser  ones  in  accord  with  ordinary  sedimentation  processes.  But, 
in  addition  to  that,  the  subsequent  changes  that  have  affected  the 
whole  rock  have  occasionally  accentuated  the  structure  by  a  ten- 


122 


NEW  YORK  STATE  MUSEUM 


dency  of  the  whole  rock  to  develop  elongate  or  fibrous  aggregates. 
It  is  probable  therefore  that  the  parting  capacity  is  in  places  con- 
siderably increased  by  the  very  process  that  has  produced  just  the 
reverse  results  in  the  more  heterogeneous  portions  of  the  beds. 

Under  a  sufficient  stress  the  rock  will  part  most  easily  along  the 
planes  where  this  foliate  or  fibrous  character  is  most  persistent. 
Even  in  these  cases,  however,  it  may  not  indicate  that  the  rock  is 
essentially  weak.  It  simply  locates  the  most  vulnerable  point  in 
the  stone.  In  many  quarries  these  streaks  are  so  abundant  that 
only  thin  slabs  can  be  obtained  —  the  disturbances  of  ordinary 
quarrying  being  sufficient  to  cause  parting.  The  deeper  portions  of 
quarries  are,  however,  much  less  subject  to  such  behavior.  In  all 
cases  the  greater  slab  development  of  the  exposed  portion  of  the 
ledge  is  an  ordinary  weathering  effect,  by  which  the  same  results 
are  obtained  slowly  and  naturally  and  more  perfectly  than  can  be 
secured  artificially  on  the  fresh  material  of  the  same  beds.  The 
expansions  and  contractions  of  changes  of  temperature,  together 
with  the  rupturing  effects  of  freezing  water  caught  in  the  pores, 
serve  finally  to  weaken  every  part  of  the  rock.  In  this  process  the 
prominent  reed  lines  give  way  so  much  in  advance  of  the  rest  of 
the  rock  that  they  develop  into  true  rifts  and  separate  slabs  appear. 
It  must  be  appreciated  that  these  ledges  have  been  exposed  an  im- 
mensely long  time  compared  with  the  probable  requirements  of  any 
engineering  structure,  and  that  this  weathering  tendency  does  not 
mean  a  speedy  disintegration  of  the  freshly  quarried  blocks.  Still 
it  is  advisable  to  avoid  as  many  sources  of  weakness  as  possible 
and  one  of  the  ways  is  to  select  ledges  where  the  stone  does  not 
have  a  reeding  tendency,  or  in  which  the  reed  lines  are  interlocked, 
or  wavy,  or  interrupted.  These  requirements  are  most  fully  met  in 
the  coarser  beds  and  especially  those  exhibiting  some  cross-bedding. 
Two  local  quarries  meet  these  demands  to  a  marked  degree. 

Strength.  The  better  qualities  of  bluestone  have  great 
strength.  Even  the  reed  lines  are  in  many  instances  stronger  and 
more  durable  than  the  regular  quality  of  some  other  sandstones 
that  are  usually  considered  suitable  building  material.  The  secret 
of  this  exceptional  strength  lies  in  the  modifications  of  texture  that 
have  resulted  from  the  alteration  and  reconstruction  of  the  mineral 
constituents.  The  breaking  up  of  the  orthoclase  feldspar,  and  the 
accompanying  changes  in  the  ferromagnesian  minerals,  have  fur- 
nished considerable  secondary  quartz,  which  has  in  part  attached 
to  the  original  quartz  grains  making  them  more  angular  and  de- 


Plate  23 


Photomicrograph  showing  structure  of  the  reeding  quality  of  "blue- 
stone.  '  Magnification  30  diameters.  Taken  to  show  tendency  to  paral- 
lelism of  elongate  grains. 


Photomicrograph  of  best  grade  coarse-grained  Milestone.  Taken  to 
snow  a  quality  m  which  the  granular  character  is  still  well  preserved. 
,  'i  !  i"'  SLains,are  quartz,  the  others  are  chiefly  feldspars  somewhat 
"Alined.  ine  close  interlocking  and  the  development  of  fibrous  or 
trayecl  structure  and  the  bending  or  wrapping  of  some  constituents  are 
secondary  effects. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  1 23 

veloping  an  interlocking  tendency  [pi.  22,  lower  figure].  At  the 
same  time  the  fibrous  sericitic  and  chloritic  aggregates  have  developed 
to  such  extent  as  to  fill  most  of  the  remaining  pores,  and  in  many 
cases  the  fibrous  extensions  have  actually  grown  partly  around  the 
adjacent  quartz  grains  [pi.  23,  lower  figure].  The  effect  has  been 
to  develop  a  silicious  binding  of  unusual  toughness.  This  combina- 
tion of  changes  has  made  a  rock  that  is  now  remarkably  well  bound 
or  interlocked  for  a  sedimentary  type. 

Durability.  First-class  stone  of  the  grades  indicated  above 
would  have  as  great  durability  as  any  stone  in  the  market,  except 
perhaps  a  true  quartzite.  With  the  exception  of  the  almost  neg- 
lectable  quantities  of  pyrite,  occasionally  found,  there  is  no  con- 
stituent prominently  susceptible  to  decay.  The  rock  as  a  whole 
mineralogically  is  stable  and  its  texture  indicates  unusual  resist- 
ance to  ordinary  disintegrating  agencies. 

General  conclusions 

From  the  microscopic  study  it  is  clear  that  the  variety  of  rock 
most  fully  meeting  the  demands  of  heavy  exposed  construction  are 
the  coarser  beds  and  those  freest  from  reed  and  shale. 

From  the  field  study  it  is  apparent  that  ledges  of  suitable  char- 
acter occur  occasionally  and  that  at  least  three  such  are  not  far 
from  the  Olive  Bridge  site. 

From  additional  explorations  it  is  certain  that  ledges  of  high 
grade  rock  occur,  and  that  the  grade  varies  rapidly  in  the  same 
bed  and  that  suitable  material  can  be  obtained  in  the  immediate 
vicinity  of  the  Ashokan  dam.  No  doubt  rock  of  equally  high 
quality  may  be  obtained  at  many  other  localities. 


CHAPTER  VI 


THE  RONDOUT  VALLEY  SECTION 

Because  of  the  fact  that  the  hydraulic  grade  of  the  Catskill  aque- 
duct as  it  approaches  the  Rondout  valley  is  nearly  500  feet  A.  T., 
an  elevation  more  than  300  feet  above  the  lowest  portions  of  the 
valley  and  more  than  200  feet  above  very  large  areas  of  it,  a  total 
width  of  more  than  4  miles  being  too  low  for  unsupported  con- 
struction of  some  kind,  and  because  of  the  general  policy  of  using 
the  pressure  tunnel  system  so  as  to  deliver  the  water  at  a  corre- 
sponding elevation  on  the  east  side  of  the  valley,  and  further 
because  of  the  very  complicated  geological  features  of  the  district 
this  section  has  been  the  seat  of  very  extensive  and  interesting 
explorations. 

Undoubtedly  a  greater  number  of  obscure  features  occur  here 
than  on  any  other  single  section  of  the  whole  aqueduct  line.  Most 
of  these  features  are  readable  from  surface  phenomena  in  general 
terms.  In  all  cases  the  indications  are  plain  enough  to  serve  as  a 
guide  to  well  directed  tests,  but  many  points  of  critical  importance 
can  not  be  determined  with  sufficient  detail  and  accuracy  of  posi- 
tion for  such  an  engineering  enterprise  without  systematic  explora- 
tion.1 The  basis  and  results  of  this  line  of  investigation  which  has 
occupied  the  greater  part  of  two  years  are  summarized  and  plotted 
in  the  following  discussion  and  charts.  The  portion  receiving 
special  study  is  in  the  vicinity  of  High  Falls. 

General  geology 

Almost  everywhere  the  surface  is  glacial  drift.  Where  outcrops 
of  bed  rock  occur  they  habitually  present  the  unsymmetrical  ridge 
appearance  usually  with  a  more  or  less  sharply  marked  escarpment 
on  one  side  and  a  gentle  slope  on  the  other.    The  strike  of  these 

1  These  explorations  belong  to  the  Esopus  division  of  the  Northern 
Aqueduct  Department.  The  earliest  reconnaissance  was  done  under  the 
direction  of  James  F.  Sanborn,  division  engineer,  who  was  subsequently 
assigned  to  geologic  work  over  a  considerable  portion  of  the  Aqueduct  line. 
The  development  of  exhaustive  explorations  and  final  construction  on  this 
division  has  been  carried  on  under  Lazarus  White,  division  engineer, 
assisted  by  Thomas  H.  Hogan.  The  division  has  been  recognized  from 
the  beginning  as  an  important  one  and  in  many  ways  one  of  the  most  com- 
plex. Thomas  C.  Brown,  now  professor  of  geology  in  Middlcbury  College, 
was  employed  for  a  year  on  this  division  during  the  later  exploratory  work. 

125 


126 


NEW  YORK  STATE  MUSEUM 


features  is  in  general  northeasterly  and  on  the  gentle  slope  is  the 
westerly  one. 

It  is  apparent  at  once  that  the  valley  bottom  is  a  complex  one 
and  that  its  history  has  been  somewhat  obscured  by  the  glacial 
deposits. 

Formations.  The  following  distinct  stratigraphic  units  are  deter- 
minable in  this  valley  every  one  of  which  will  be  cut  by  the  tunnel 
beginning  at  the  west  side  with  the  youngest  formations : 

Feet 

Hamilton  and  Marcellus  flags  and  shales   700-+ 

Onondaga  limestone  t   200 

Esopus  gritty  shales   800-+ 

Port  Ewen  shaley  limestone  including  the  Oriskany  transition   250-+ 

Becraft  crystalline  limestone   75 

New  Scotland  shaley  limestone   100 

Coeymans  limestone    75 

Manlius  limestone  including  Rosendale,  Cobleskill,  and  the  cement 

beds   •   100-+ 

Binnewater  sandstone    50 

High  Falls  shale  including  small  limestone  layers   75 

Shawangunk  conglomerate  250  to  350 

Hudson  River  slates  —  thickness  unknown ;  probably  more  than  2000 

Approximately  4775 

These  occur  in  belts  in  succession  more  or  less  regularly  from 
west  to  east.  Most  of  the  formations  are  quite  uniform  in  the 
Rondout  valley.  The  Shawangunk  conglomerate  is  probably  more 
variable  than  any  other  as  shown  by  borings.  Because  of  this 
general  persistence  of  formation  it  is  possible  to  estimate  approxi- 
mately the  depth  at  which  any  particular  lower  member  lies  if  some 
starting  point  can  be  identified.  [For  detailed  description  of  the 
formation,  see  pt  1] 

Structure.  The  principal  irregularities  are  structural,  rather 
than  stratigraphic.  The  region  on  the  west  side  of  the  valley,  the 
margin  of  the  Catskills,  is  but  slightly  disturbed  and  lies  very  flat, 
but  the  region  on  the  east  side,  the  Shawangunk  mountain  range 
and  the  cement  district,  has  an  extremely  complicated  structure. 
The  Rondout  valley,  lying  between  them,  is  a  transitional  zone  and 
passes  from  gentle  dip  slopes  and  folds  in  the  westerly  side  to 
more  frequent  folds  and  thrust  faults  on  the  easterly  side.  In  at 
least  two  thirds  of  the  valley  it  would  appear  from  surface  evi- 
dence alone  that  the  formations  would  dip  uniformly  westward,  the 
only  suspicion  of  additional  complication  being  given  by  an  occa- 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


127 


sional  minor  fold  seen  in  the  river  gorge  or  an  escarpment  where 
the  sedimentary  character  alone  would  hardly  account  for  it  [see 
pi.  24,  High  Falls].  Explorations  have  shown  that  the  evidence  of 
the  minor  structures  is  reliable  and  that  disturbances  occur  at  some 
places  even  to  the  extreme  western  margin. 

Physiography.  In  spite  of  the  drift  cover  which  obscures 
many  original  inequalities  it  is  readily  seen  that  the  prevalence  of 
the  gentle  westerly  dip  over  most  of  the  area,  together  with  the 
succession  of  so  many  different  beds  of  varying  resistance  to  ero- 
sion, have  allowed  the  development  of  a  succession  of  long  dip 
slopes  and  steep  escarpments  on  a  more  pronounced  scale  than  the 
present  topography  shows.  It  is  clear  that  the  Rondout  is  really  a 
series  of  these  unsymmetrical  valleys.  The  principal  large  dip 
slopes  are  formed  by  the  Shawangunk  conglomerate  and  the  Onon- 
daga limestone.  In  each  case  an  original  stream  had  adjusted  its 
course  fully  to  the  structure  and  was  shifting  slowly  by  the  sapping 
process  to  the  west  against  the  opposing  edges  of  the  overlying 
strata  which  form  the  bordering  escarpment.  One  of  these  unsym- 
metrical valleys  lies  along  the  easterly  base  of  the  Hamilton  escarp- 
ment and  is  continuous  with  the  lower  course  of  Esopus  creek 
farther  to  the  north.  In  the  area  under  special  study  it  is  not 
occupied  by  a  stream  now  but  is  filled  with  glacial  drift  so  com- 
pletely that  the  original  stream  has  been  evicted.  It  is  evident, 
however,  from  computations  based  upon  the  average  dip  of  the 
slope  carried  to  the  base  of  the  escarpment  that  the  bed  rock  floor 
ought  to  be  from  200  to  300  feet  below  the  present  surface  in  the 
deepest  portion.  Borings  have  proven  this  to  be  the  case  both 
along  the  present  line  near  Kripplebush  and  also  on  the  first  trial 
line  across  the  Esopus  at  Hurley. 

The  same  thing  is  true  near  High  Falls  in  the  center  of  the  valley 
where  Shawangunk  conglomerate  forms  the  dip  slope  and  the 
escarpment  is  formed  by  the  Helderberg  limestones.  In  this  case 
the  drift  filling  is  very  deep  also,  and  Rondout  creek  flows  upon  it 
quite  independent  of  rock  structure  except  where  it  has  cut  across 
the  margin  as  at  High  Falls. 

In  the  eastern  half  of  the  valley  the  hard  Shawangunk  conglom- 
erate forms  the  chief  rock  floor  and  largely  controls  the  contour  by 
its  own  foldings  and  other  displacements.  Thus  the  Coxing  kill 
tributary  valley  lies  in  a  syncline  of  the  conglomerate  with  occa- 
sional remnants  of  overlying  beds  as  outliers  adding  some  variety 
to  the  form.    The  Shawangunk  mountains,  as  a  physiographic 


128 


NEW  YORK  STATE  MUSEUM 


feature,  owe  their  present  elevation  chiefly  to  the  resistance  of  this 
conglomerate  which  serves  as  a  protective  member  among  the 
formations. 

On  the  west  side,  the  foothills  of  the  Catskills  form  a  part  of 
the  cuesta  developed  by  the  erosion  of  Paleozoic  sediments,  the 
inface  coinciding  with  the  escarpment  along  the  lower  Esopus  and 
Rondout  valleys  at  this  point. 

It  is  certain  therefore  that  the  drainage  of  the  Rondout  valley 
before  the  Ice  age  differed  materially  from  the  present  lines.  A 
stream,  probably  the  original  Rondout,  followed  near  the  western 
margin  of  the  valley  and  joined  the  Esopus  as  it  emerged  from  the 
Hamilton  escarpment  to  turn  northeast.  Another  which  had  cut 
somewhat  deeper  occupied  the  central  portion  of  the  valley  and 
probably  joined  the  Esopus  at  some  point  farther  north  —  its  lower 
course  is  not  explored. 

Practical  questions 

The  chief  practical  questions  to  be  given  as  full  answers  as  pos- 
sible are: 

1  At  what  depth  must  the  aqueduct  tunnel  be  placed  in  order 
to  be  everywhere  in  substantial  bed  rock  with  sufficient  cover  to  be 
safe  ? 

2  Where  are  the  most  critical  places  —  those  whose  geologic 
characters  are  such  as  to  demand  exploration  ?  And  at  the  same 
time  which  sections  may  be  safely  left  without  testing? 

3  What  is  the  rock  structure  and  condition?  And  are. there  rea- 
sons for  believing  that  the  tunnel  plan  is  not  feasible  at  this  point. 
If  so,  where  can  a  better  one  be  found? 

4  What  is  the  character  of  underground  circulation  of  water? 

5  What  formations  will  be  cut  at  the  different  points  and  which 
should  be  favored  or  avoided  wherever  possible? 

From  the  fact  that  the  present  Rondout  flows  across  solid  ledges 
at  High  Falls  and  at  Rosendale  from  ioo  to  200  feet  above  the 
known  rock  floor  of  the  preglacial  gorge  where  explored  it  is  clear 
that  the  present  course  is  entirely  different  from  the  original.  The 
Coxing  kill,  the  third  and  most  easterly  of  these  streams  is  not  so 
much  disturbed  although  it  also  is  shifted. 

It  is  worth  noting  that  the  streams  of  this  valley  together  with 
the  lower  Esopus  and  the  Wallkill  river  have  become  so  completely 
adjusted  to  the  rock  structure  that  they  all  flow  up  the  larger 
Hudson  valley,  of  which  all  form  a  part,  and  join  the  master  stream 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


129 


at  an  obtuse  instead  of  the  usual  acute  angle.  They  are  essentially 
retrograde  streams. 

Explorations.  Systematic  explorations  and  tests  are  repre- 
sented chiefly  by  drill  borings  through  drift  into  the  rx>ck  floor. 
These  were  supplemented  by  two  test  tunnels  for  working  character 
of  material  and  a  series  of  tests  on  the  behavior  of  certain  of  the 
drill  holes,  together  with  other  tests  on  material.  The  results  are 
embodied  in  the  accompanying  cross  sections  and  the  additional 
discussion  of  special  features. 

Detail  of  local  sections 

Kripplebush  section.  This  from  the  first  was  regarded  as  one 
of  the  critical  sections  because  of  the  buried  gorge  along  the  base 
of  the  Hamilton  escarpment  and  because  of  the  doubt  as  to  the 
behavior  of  the  Onondaga  limestone.  On  the  accompanying  section 
the  borings  are  plotted  and  the  structure  as  now  interpreted  is 
indicated.  The  dip  slope  formed  by  the  Onondaga  limestone  is 
covered  by  200  to  250  feet  of  drift,  mostly  modified  drift.  The 
strong  valley  character  of  the  rock  floor  is  almost  wholly  obscured 
by  the  glacial  deposits  and  the  present  brook,  an  insignificant  stream 
compared  to  the  preglacial  one,  occupies  a  position  above  the  escarp- 
ment instead  of  above  the  old  channel. 

After  a  couple  of  the  central  holes  were  finished,  it  became  appar- 
ent that  the  structure  is  not  nearly  so  simple  at  this  point  as  the 
general  surface  features  would  lead  one  to  expect.  It  was  clear 
that  a  simple  dip  such  as  was  proven  to  prevail  on  the  dip  slope 
would  not  account  for  the  much  greater  depth  attained  by  it  in 
the  vicinity  of  station  500.  The  discovery  of  this  additional  feature 
raised  two  questions:  (1)  Is  the  structure  a  flexure  or  is  it  a  fault, 
and  if  a  fault  whether  normal  or  thrust,  and  (2)  what  is  the  prob- 
able effect  of  this  structure  on  the  position  and  depth  of  the  pre- 
glacial gorge  ? 

The  habit  of  the  district  immediately  east  of  the  valley  would 
support  the  theory  of  a  thrust  fault.  The  nature  of  the  immediate 
area  would  suggest  a  simple  flexure  while  it  is  manifestly  possible 
that  a  normal  fault  could  easily  occur.    Later  explorations1  have 

1  Since  the  above  was  written  the  tunnel  has  been  completed  through  the 
Kripplebush  section.  Although  faulting  is  indicated  by  the  borings  and 
actual  occurrence  of  the  beds  it  is  very  difficult  to  find  the  fault.  A  part 
of  the  displacement  is  accomplished  by  the  steepening  of  the  dip  but  this 
will  not  account  for  more  than  half  of  it. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  131 


tested  this  zone  so  well  that  it  is  practically  certain  that  the  feature 
must  he  regarded  as  a  fault  of  some  type  with  a  displacement 
of  nearly  200  feet.  The  striking  physiographic  feature  is  the 
development  and  preservation  of  the  escarpment  on  the  downthrow 
side.  This  occurrence  is  certainly  a  very  unusual  case  in  that 
regard  [see  fig.  19]. 

Because  of  the  intention  to  construct  the  tunnel  deep  enough  in 
bed  rock  to  reach  safe  rock  conditions  the  question  of  depth  of 
buried  gorge  becomes  an  important  one.  As  soon  as  it  was  dis- 
covered that  a  fault  existed  there  the  problem  became  of  sufficient 
prominence  to  demand  more  detailed  exploration.  If  the  faulting 
is  accompanied  by  a  broken  zone  in  condition  favorable  to  more 
ready  erosion,  it  would  be  possible  that  the  original  stream  in  work- 
ing down  this  dip  slope  might  become  entrenched  in  the  fault  zone 
and  at  that  point  begin  to  cut  a  narrow  gorge  instead  of  continuing 
the  sapping  process.  In  fact,  it  would  undoubtedly  do  this  very 
thing  if  there  is  such  a  crushed  zone  of  any  consequence  and  if  the 
erosion  process  were  allowed  to  continue  long  after  reaching  this 
critical  point. 

As  a  matter  of  fact  explorations  have  shown  that  there  is  a  thin 
layer  of  Hamilton  shales  still  remaining  on  the  Onondaga  and  the 
deepest  point  found  is  on  the  Hamilton  shales  side.  These  facts 
in  connection  with  the  failure  to  find  any  deep  notch  indicate  that 
there  is  probably  no  zone  of  much  greater  weakness  than  the  shale 
member  itself.  It  is  reasonable  to  conclude  that  the  rock  floor  can 
be  safely  regarded  as  not  much  lower  than  88  feet  A.  T.  and  that  the 
rock  condition  is  not  especially  bad  for  tunnel  construction1  even  in 
the  fault  zone. 

Rondout  creek  section.  This  is  the  central  portion  of  the 
valley  including  the  depression  occupied  by  the  present  Rondout 
and  the  exposed  edges  of  the  series  of  shales  and  Helderberg  lime- 
stone. The  repetition  of  the  dip  slope  and  escarpment,  together 
with  the  heavy  drift  filling  and  the  occurrence  of  so  many  forma- 
tions together  make  this  an  important  section.  All  formations  from 
the  Shawangunk  conglomerate  to  the  Port  Ewen  shaly  limestone 
occur  at  this  point,  and  although  there  is  little  outward  evidence  of 
disturbance  it  is  certain  that  whatever  difficulty  is  to  be  found  in 
this  variable  series  is  likely  to  be  met  here.  It  is  therefore  a  sec- 
tion that  requires  exploration  both  for  depth  of  preglacial  channel 
and  for  quality  of  rock. 


In  construction  this  ground  has  proven  to  be  good  and  sound  throughout. 

5 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  I33 


All  of  the  formations  dip  westward  wherever  exposed,  but  the 
dips  vary  somewhat,  nearly  all  being  of  low  angle.  Occasional 
minor  inequalities  of  the  nature  of  small  rolls  may  be  seen,  as,  for 
example,  the  small  fold  in  the  gorge  at  High  Falls  [see  pi.  24]. 

Explorations  have  shown,  as  indicated  on  the  accompanying  cross 
section  [fig.  20],  that  there  is  a  deeper  buried  gorge  here  than  at 
Kripplebush.  The  deepest  point  discovered  is  a  few  feet  below  tide 
level.  The  escarpment  is  steep  and  is  formed  by  the  Coeymans  and 
New  Scotland  formations.  The  dip  slope  is  Shawangunk  conglom- 
erate, High  Falls  shale  and  Binnewater  sandstone,  with  the  Manlius 
limestone  forming  the  floor. 

Identification  of  the  drill  cores  which  penetrate  the  limestone 
indicate  that  the  dip  slope  is  reversed  on  the  west  side  of  the  gorge 
and  that  the  stream  had  really  reached  about  the  axis  of  the  trough. 
A  discrepancy  in  thicknesses  and  depths  in  hole  no.  34  by  which  it 
appeared  that  the  Coeymans  formation  was  almost  twice  as  thick 
as  usual  and  that  it  contained  a  broken  or  crushed  zone  leads  to 
the  interpretation  that  there  is  a  small  thrust  fault  here  which  re- 
peats the  formation  as  shown  on  the  accompanying  cross  section. 

Instead  of  a  uniform  westerly  dip  of  all  formations  from  the 
Rondout  westward  it  is  proven  that  minor  anticlinal  rolls  and  even 
thrust  faults,  as  in  this  case,  or  such  faults  as  in  the  Kripplebush 
case  are  not  to  be  excluded. 

This  structural  relation  has  a  direct  bearing  upon  the  question  of 
the  thickness  of  the  Esopus  shales.  The  Esopus  is  certainly  not  so 
thick  as  would  otherwise  be  supposed,  by  200  or  300  feet  at  the 
least.  The  true  thickness  is  still  an  unknown  quantity  (estimated 
at  800  feet). 

It  is  clear  that  the  aqueduct  tunnel  will  have  to  be  constructed  a 
considerable  depth  below  sea  level  at  this  section,  probably  not  less 
than  minus  150  feet,1  even  if  the  character  of  the  formations  be 
neglected. 

But  the  character  or  quality  of  these  formations  in  view  of  their 
structural  relation  constitutes  the  chief  problem.  Because  of  the 
fact  that  every  structure  reaches  the  surface  and  eventually  dips 
gently  to  the  west  in  such  manner  as  to  encourage  water  circulation, 
their  water-carrying  capacity  or  general  porosity  becomes  of  great 
importance.  A  great  capacity  is  all  the  more  serious  because  of 
the  heavy  drift  cover  within  the  abandoned  gorge,  on  top  of  which 

1  This  portion  of  the  tunnel  and  its  continuation  south  to  the  Shawangunk 
range  has  been  constructed  at  250  feet  below  sea  level. 


134 


NEW  YORK  STATE  MUSEUM 


the  stream  flows  and  which  constitutes  essentially  an  unlimited 
storage  reservoir  to  feed  underground  circulation.  This  is  all  the 
mor-e  true  if  crush  zones  are  extensively  developed  as  accompani- 
ments of  the  faulting. 

In  general  as  to  perviousness  the  indications  are  somewhat  ob- 
scure. But  the  data  now  obtained  seem  to  prove  that  all  the  for- 
mations except  the  Binnewater  sandstone  and  the  High  Falls  shale 
are  compact  and  fairly  impervious  along  the  bedding  lines.  Only 
where  crevices  have  formed  or  where  crushing  occurs  is  there 
likely  to  be  heavy  circulation.  This  is  all  the  more  important  since 
so  many  of  the  beds  are  limestones  known  to  be  readily  soluble  in 
circulating  water.  One  of  these  limestones,  the  Manlius,  exhibits 
occasional  large  open  solution  joints  at  the  surface  —  so  large  that 
a  surface  stream  disappears  entirely  at  the  so  called  "  Pompey's 
cave  "  and  joins  the  subterranean  circulation.  But  such  caves  are 
probably  limited  to  the  surface. 

It  is  near  this  point,  however,  that  one  of  the  earlier  borings  at 
one  side  of  the  present  line  discovered  very  soft  ground  at  a  depth 
of  about  sea  level,  i.  e.  over  200  feet  below  the  present  surface, 
which  shows  that  similar  conditions  prevail  at  certain  points  to 
great  depth. 

Pumping  tests  made  on  hole  no.  32  in  an  attempt  to  establish 
some  data  on  the  inflow  of  water  gave  very  interesting  results. 
These  tests  were  very  thorough.  It  was  proven  that  the  water  was 
supplied  in  apparently  inexhaustible  quantity  at  maximum  pumping 
capacity,  which  was  ninety  gallons  per  minute.  Futhermore,  the 
chief  inflow  seemed  to  be  from  the  Binnewater  and  High  Falls 
formations  as  was  to  be  expected.  Whether  a  crush  zone  allowing 
free  circulation  is  furnishing  a  portion  of  this  supply  or  whether 
the  whole  inflow  represents  the  normal  porosity  condition  of  these 
formations  is  not  yet  proven.1 

Other  porosity  tests  have  been  made  in  such  way  as  to  locate 
and  measure  this  factor  [see  later  discussion].  Hole  no.  10  shows 
an  artesian  overflow  that  comes  from  the  Binnewater  sandstone. 
A  working  shaft  has  been  put  down  also  in  the  vicinity  of  hole 
no.  32  and  at  the  same  depth  found  an  enormous  inflow  of  water 
which  drowned  out  operations  for  a  time.  The  lateral  supply  in 
this  case  has  been  reduced  by  introducing  a  thin  cement  grouting 
through  holes  bored  in  the  surrounding  rock  from  the  surface. 

Holes  no.  12  and  no.  14  also  show  an  artesian  flow,  but  both  are 


1  In  construction  the  Binnewater  sandstone  has  been  found  very  wet. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


135 


"shallow  holes  and  the  supply  comes  from  near  the  contact  between 
High  Falls  shale  and  Shawangunk  conglomerate. 
/  It  is  certain  from  these  observations  and  tests  therefore  that  the 
Binnewater  sandstone  and  High  Falls  shale  are  more  porous  than 
the  other  formations,  and  because  of  the  serious  difficulties  arising 
from  so  heavy  inflow  of  water  from  them  the  tunnel  grade  should 
be  shifted  so  as  to  avoid  these  formations  as  much  as  possible.  A 
comparison  of  the  accompanying  cross  section,  which  is  drawn  to 
scale  [fig.  20J ,  will  show  that  a  tunnel  on  one  level  would  neces- 
sarily run  for  a  long  distance  in  these  beds  because  of  the  gentle 
syncline.  Furthermore,  they  lie  at  about  the  depth  that  would 
otherwise  be  a  safe  depth  below  the  buried  gorge.  But  a  tunnel 
with  a  step-down,  i.  e.  one  run  at  two  different  levels  could  avoid 
most  of  this  poor  ground.  By  approaching  at  a  level  of  about  —  50 
feet  or  —  100  feet  in  the  limestone  beds  to  station  600  (hole  no.  34), 
then  stepping  down  to  —  250  feet,  the  line  in  a  very  short  distance 
crosses  these  two  porous  formations  and  enters  the  Shawangunk 
conglomerate  which  is  more  substantial,  and,  all  things  considered, 
one  that  seems  most  advantageous  for  successful  construction.  It 
will  have  to  maintain  a  head  of  more  than  700  feet  as  the  difference 
between  hydraulic  grade  and  the  tunnel  level  in  this  section.  Under 
these  conditions  rock  quality  and  condition  are  of  greatest  impor- 
tance and  there  is  no  doubt  about  the  advisability  of  avoiding  the 
poorest  formations  in  some  such  manner. 

Coxing  kill  section.  On  the  line  of  exploration  the  Coxing 
kill  flows  over  Shawangunk  conglomerate  and  High  Falls  shale. 
Both  dip  plainly  eastward,  and  a  hole  no.  1 1  located  on  the  east 
side  of  the  brook  penetrates  about  70  feet  of  drift  and  shale.  But 
only  a  hundred  feet  to  the  east  Shawangunk  conglomerate  outcrops 
at  the  surface  dipping  the  same  way.  It  is  certain  therefore  that 
a  fault  occurs  here.  The  dip  of  the  fault  plane  is  indeterminate 
from  the  surface,  but  the  relations  and  surroundings  indicate  a 
fault  of  the  thrust  type. 

Later  explorations  indicate  that  the  fault  plane  is  rather  flat 
[see  cross  section  fig.  21]  so  that  the  shales  are  repeated  above 
and  below  a  tongue  of  conglomerate.  Boring  no.  1 1  has  also  an 
artesian  flow  of  considerable  volume  coming  from  near  the  bottom 
of  the  conglomerate.    It  is  a  mineral  water. 

The  chief  importance  of  this  section  as  a  problem  in  applied 
geology  lies  in  the  influence  of  the  fault  and  the  maximum  de- 
pression of  the  conglomerate.  If  the  tunnel,  which  enters  Hud- 
son River  slates  at  the  Rondout  creek  section  at  — 250  feet  can 
be  kept  within  that  formation  throughout  the  rest  of  its  course, 


i36 


NEW  YORK  STATE  MUSEUM 


there  is  no  doubt  that  an  advantage  will  be  gained  both  in  the 
greater  imperviousness  of  the  rock  and  the  greater  case  of  pene- 
tration. Wherever  the  conglomerate  is  undisturbed  it  is  perfectly 
good,  but  where  broken  the  crevices  are  but  imperfectly  healed 
and  circulation  is  unhindered.  It  would  therefore  be  desirable  to 
know  whether  at  —  250  feet  the  whole  of  the  downward  wedge  of 
Shawangunk  could  be  avoided.  The  borings  indicate  a  thickness 
of  Shawangunk  of  345  feet  in  hole  no.  11  where  it  is  cut  at  a 
small  angle,  and  a  thickness  of  409  feet  in  hole  no.  36  where  it  prob- 
ably lies  pretty  flat.     This  greater  thickness  together  with  the 


finding  of  crushed  rock  at  about  the  —  100  foot  level  leads  to  the 
conclusion  that  the  formation  is  overthickened  here  by  the  thrust 
fault  to  the  extent  probably  of  about  75  feet.  The  true  thick- 
ness of  the  formation  at  this  point  is  doubtless  more  nearly  300 
feet  than  either  of  the  figures  obtained  directly  from  the  two 
holes.  If  this  interpretation  is  used  as  the  basis  of  plotting  a 
cross  section  [sec  accompanying  cross  section]  it  is  apparent  that 
the  conglomerate  should  not  be  expected  to  extend  more  than  a 
few  hundred  feet  east  of  hole  no.  36  and  it  probably  does  not  reach 
a  much  greater  depth  than  the  — 236  feet  represented  as  its  base- 
in  that  boring.1 

1  Construction  of  the  tunnel  has  progressed  far  enough  through  this  sec- 
tion to  prove  that  the  Shawangunk  formation'  does  not  reach  much  lower. 
It  forms  the  roof  of  the  tunnel  for  some  considerable  distance  but  does  not 
come  down  into  the  tunnel  more  than  a  foot  or  two. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


137 


Shawangunk  overthrust.  At  the  extreme  eastern  side  of  the 
Rondout  valley  near  the  point  where  the  surface  reaches  hydraulic 
grade  again,  the  surface  outcrops  pass  from  High  Falls  shale  to 
Shawangunk  conglomerate  to  Hudson  River  shale  in  the  normal 
order  but  with  entirely  too  small  an  area  of  conglomerate  consid- 
ering the  character  of  the  formations.  The  higher  ground  is  all 
Hudson  River  in  the  vicinity,  and  there  is  abundant  evidence  of 
crushing  and  disturbance.  It  is  evident  that  a  thrust  fault  is  again 
encountered  here,  one  of  sufficient  throw  to  bring  the  Hudson  River 
slates  above  the  Shawangunk  conglomerate  —  probably  a  lateral 
displacement  of  very  great  extent.  Explorations  have  fully  proven 
the  existence  of  this  fault.  The  accompanying  diagram  shows  a 
cross  section  as  now  outlined  by  complete  penetration  of  two 
borings. 

Two  trial  tunnels  were  run  to  test  working  quality  of  Hudson 
River  slates  compared  to  Shawangunk  conglomerate  at  this  locality. 
Both  are  within  the  influence  of  the  fault  zone.  Both  are  there- 
fore more  broken  than  the  normal  with  the  result  that  the  Hudson 
River  slates  probably  show  poorer  condition  than  usual  and  more 
troublesome  working,  while  Shawangunk  conglomerate  probably 
shows  easier  working  than  usual.  It  is  believed  that  normally  the 
two  rocks  would  present  a  greater  difference  than  was  found  in 
this  test. 

Special  features 
Several  questions,  some  of  which  have  a  practical  bearing,  have 
been  raised  as  separate  features  during  the  exploration  of  the 
Rondout  valley. 

Caves.  One  of  these  is  in  regard  to  the  possible  existence  of 
underground  caverns.  This  was  given  a  special  prominence  early 
in  the  work  by  the  experience  of  one  of  the  drills.  After  pene- 
trating the  limestone  series  near  High  Falls  to  a  depth  of  over 
200  feet,  the  drill  seemed  to  leave  the  rock  and  enter  a  space 
allowing  the  rods  to  drop  28  feet  before  being  arrested  by  solid 
material.  The  further  attempt  to  work  in  this  hole  resulted  in 
the  breaking  of  the  rod  doAvn  at  this  point  and  the  subsequent 
failure  to  recover  the  diamond  bit  which  is  still  in  the  bottom  of 
the  hole.  The  question  is  as  to  the  meaning  of  this  occurrence. 
Is  it  a  cavern? 

"  Pompey's  cave  "  has  been  referred  to  in  an  earlier  paragraph. 
This  is  clearly  not  much  of  a  cave.  It  is  essentially  an  enlarged 
joint  or  series  of  joints  by  solution  along  the  bed  of  a  surface 


138 


NEW  YORK  STATE  MUSEUM 


stream  to  such  extent  that  the  stream  normally  at  present  has  be- 
come subterranean.  It  is  the  writer's  opinion  that  the  case  en- 
countered by  the  drill  boring  is  similar.  The  apparent  cavern  is 
probably  a  slightly  enlarged  joint  along  a  line  of  somewhat  abun- 
dant underground  circulation  and  perhaps  associated  with  some 
crush  zone  developed  by  the  small  faulting  known  to  occur  in  this 
immediate  vicinity.  It  is  probably  not  entirely  empty  but  contains 
residuary  clay,  and  in  all  likelihood  is  very  narrow  and  not  exactly 
vertical,  so  that  the  drill  rods  were  bent  out  of  their  normal  course 
and  wedged  into  the  lower  part  of  the  crevice.  Smaller  spaces 
of  this  sort  were  encountered  at  a  few  other  points.1 

These  occurrences  seem  to  indicate  that  the  limestone  beds  yield 
rather  readily  to  solution  by  underground  water,  and  that  this  cir- 
culation has  been  at  one  time  active  to  at  least  50  feet  below  pres- 
ent sea  level.  With  present  ground  water  level  nearly  200  feet 
above  sea  level  it  is  extremely  unlikely  that  any  such  action  is 
going  on  at  so  great  depth.  The  occurrence  is  therefore  strongly 
corroborative  of  former  greater  continental  elevation  when  the 
deep  stream  gorges,  now  buried,  were  being  made.  These  deeper 
caverns  or  solution  joints  probably  date  from  that  epoch. 

Imperviousness  and  insolubility.  The  question  of  impervious- 
ness  and  closely  associated  with  it  that  of  solubility,  is  of  great 
practical  importance  in  this  particular  work.  The  immense  pres- 
sure under  which  the  tunnel  will  be  placed  in  crossing  this  valley 
makes  it  impossible  to  construct  a  water-tight  lining.  Everywhere 
much  depends  upon  the  rock  walls  to  help  hold  the  water  from 
sjeriotis  iloss.  Wherever  the  rock  is  fairly  impervious  except 
occasional  crevices  or  joints  they  can  be  grouted  and  safeguarded 
satisfactorily.  But  where  a  formation  is  of  general  porosity  this 
can  not  be  so  successfully  done.  Even  more  difficult  to  handle  is 
the  rock  wall  which  is  soluble  and  which  therefore  with  enforced 
seepage  may  tend  to  become  progressively  more  porous.  That  this 
consideration  is  not  wholly  theoretical  is  shown  very  forcibly  by  the 
Thirlmere  aqueduct  of  the  Manchester  (England)  Waterworks. 
In  that  case  a  3  mile  section  was  built  through  limestone  country 
using  the  same  local  limestone  for  concrete  aggregate.  Although 

1  In  constructing  the  tunnel  several  clay-filled  spaces  have  been  discovered 
in  the  same  vicinity  at  elevation — 100.  One  of  these  extended  vertically  with 
a  width  of  1  to  2  feet  and  from  it  a  great  mass  of  mud  ran  into  the  tunnel. 
At  one  point  it  was  connected  with  a  horizontal  space  of  the  same  kind 
extending  15  feet.  It  can  be  seen  that  the  original  crevices  have  been  en- 
larged by  water  and  that  they  were  originally  formed  during  faulting. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


139 


this  concrete  was  mixed  as  rich  as  1  part  cement  to  5  parts  aggre- 
gate and  the  work  was  well  done,  excessive  leakage  reaching  a  total 
of  1,250,000  imperial  gallons  per  day  was  developed  within  a  year. 
It  was  found  that  the  limestone  fragments  of  the  aggregate  were 
corroded  forming  holes  through  the  lining  of  the  aqueduct  and  that 
these  holes  actually  enlarged  outward.  All  this  was  done  under  cut 
and  cover  conditions  with  not  more  than  a  6  or  7  foot  head  on  the 
bottom  of  the  aqueduct. 

In  the  Rondout  valley,  the  aqueduct  will  cut  no  less  than  6  lime- 
stone beds  in  all  cases  under  great  pressure.  This  fact  will  in  all 
probability  tend  to  increase  the  action.  But,  of  course,  some  of  the 
beds  may  not  yield  so  readily  to  solution.  Tests  made  thus  far, 
however,  indicate  that  all  are  attacked  in  water.  Considering  these 
facts  it  seems  desirable,  so  far  as  possible,  to  avoid  the  limestone 
beds  wherever  rock  of  greater  resistance  to  solution  can  be  reached, 
and  further  it  is  equally  desirable  to  use  a  more  resistant  rock  for 
the  lining  concrete.  So  long,  however,  as  the  formation  is  not  very 
pervious  so  that  a  new  circulation  could  not  be  established  by  the 
escaping  water  there  would  be  little  harmful  effect. 

An  average  of  five  analyses  of  the  Thirlmere  limestone,  different 
varieties  of  the  same  formation,  gives  the  following: 

Insoluble  silicious  matter   2.772% 

Alumina    and  iron  oxid  Al203+Fe203   0.276 

Lime,  CaO    53.676 

Magnesia,  MgO    .390 

Carbonic  anhydrid,  COs   42.248 

Total   99.362 

Estimated  calcium  carbonate,  CaC03  —  95.85,^ 
The  limestone  is  fossiliferous. 

Suitable  analysis  of  the  limestones  of  the  Rondout  valley  are  not 
recorded  in  sufficient  numbers.   But  these  are  a  few,  as  given  below. 


BECRAFT  LIMESTONE. 


At  Hudson 

At  Rondout  At  Wilbur  (av'ge  of  2) 


Si02 

A1202 

Fe2Os 

CaO 

MgO 

co3 . 


3.87%     7.10%  1.865% 

1.07        2.50  .818 

1,34         1-65  1-185 

54-H  45-32  51-375 

tr             tr  2.870 

40.60  39.10  40-795 


Total    100.99 

Corresponding  to  total  calcium  carbonate..  96.62 


95-67 
80.75 


98.908 
91-74 


'1 


140 


NEW  YORK  STATE  MUSEUM 


This  is  a  limestone  that  in  composition  and  structure  at  the 
Rondout  valley  is  apparently  not  very  different  in  quality  from  the 
Thirlmere  rock.  Analyses  of  the  cement  rock  show  less  similarity 
but  observations  indicate  that  it  is  also  attacked. 

It  is  probable  from  all  these  facts  that  the  shales  and  conglomer- 
ates are  better  quality  of  wall  than  the  limestones. 

A  very  acute  observation  along  this  line  by  Dr  Thomas  C.  Brown 
while  employed  on  the  staff  of  the  Board  of  Water  Supply  is  of 
special  interest.  In  studying  local  conditions  he  noticed  that  the 
limestone  blocks  used  in  building  the  old  Delaware  and  Hudson 
(D.  &  H.)  canal  showed  the  effect  of  contact  with  the  water.  The 
best  place  for  measurable  data  seemed  to  be  around  the  old  locks 
where  squared  and  evenly  trimmed  blocks  had  been  used.  These 
were,  during  the  years  of  its  use,  from  1825  (approximately  35  to 
40  years)  subject  to  the  action  of  water  flowing  or  standing  in 
direct  contact.  The  coigns  of  the  locks,  which  were  without  doubt 
freshly  and  well  cut  when  laid,  are  now  etched  till  the  fossils  and 
other  cherty  constituents  stand  out  from  one  eighth  to  one  half 
inch  beyond  the  general  block  surface,  and  in  some  cases  the  pits 
are  an  inch  deep.  That  this  etching  is  due  to  the  water  rather 
than  to  exposure  to  weather  is  shown  by  the  lack  of  such  extensive 
action  on  blocks  used  in  houses  and  exposed  a  much  longer  time. 
Blocks  representing  the  Manlius  and  Coeymans  were  identified. 
But  there  is  no  reasonable  doubt  that  others  would  be  similarly 
affected.   On  some  it  would  be  less  easily  detected. 

On  account  of  the  disturbances  another  factor  is  introduced. 
Rocks  which  readily  heal  their  fractures  are  likely  to  furnish  better 
ground,  i.  e.  more  free  from  water  circulation  especially,  than  rocks 
more  brittle  and  slow  to  heal.  Therefore  in  this  district  the  shales 
and  slates  such  as  the  Hudson  River  series  and  the  Esopus  and 
Hamilton  shales  are  the  best  ground,  while  the  Binnewater  sand- 
stone is  the  poorest. 

Cross  sections.  Probably  in  no  region  of  like  extent  is  it 
possible  to>  construct  a  geologic  cross  section  of  so  many  complex 
features  so  accurately  as  can  now  be  done  of  the  Rondout  valley 
along  the  aqueduct  line.  The  section  is  known  or  can  be  computed 
to  a  total  depth  below  the  surface  of  1000  feet,  including  12  dis- 
tinct formations,  so  closely  that  any  bed  or  contact  can  be  located 
within  a  few  feet  at  any  point  throughout  a  total  distance  of  over 
4  miles. 

The  accompanying  cross  section  contains  as  much  of  this  data 
as  is  now  available  [fig.  22]. 


1? 

!  * 

ij 

s  5 

*  it 

1    ,  {•]  i 

.J 

$  1  Mill 

\ 

'f 

1 

 Vf-  — i  i-  - 

/of  m 

6i  I 


■■' 

r 

I 

1 

GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


I4I 


Rondout  siphon  statistics 

1  Total  borings  on  the  siphon  line.  Three  different  boring 
equipments  have  been  used  owned  by  different  parties  and  records 
have  heen  kept  so  that  the  work  of  each  can  be  followed  or  com- 
pared with  the  others. 

On  this  division  the  Board  of  Water  Supply  owned  and  operated 
one  machine  with  their  own  men,  another  equipment  was  owned 
and  operated  by  C.  H.  McCarthy,  while  a  third  which  finally  did 
a  majority  of  the  work,  belonged  to  Sprague  &  Henwood,  Con- 
tractors, of  Scranton,  Pa. 

The  totals  of  different  general  types  of  material  penetrated  by 
these  machines  are  as  follows : 


Feet 

Feet 

Total 

Per  cent 

of 

of 

feet  of 

of  core 

drift 

rock 

depth 

saved 

a  B.  W.  S.  Equipment  

  1740. 5 

2175-5 

39l6 

89.4 

b  Sprague  &  Henwood  

  3647 

6831 

IO478 

60.04 

  181 

1228 

I409 

78.1 

The  average  saving  of  core  by  all  machines,  cutting  all  kinds  of 
bed  rock  was  75.96^ 

2  Core  recovery  from  various  strata.  So  nearly  as  can  be 
done  the  strata  represented  in  the  drill  cores  have  been  identified 
and  summarized  as  to  total  penetration  and  core  saving.  The  core 
saving  is  a  factor  of  prime  importance  in  judging  of  the  quality  of 
rock  and  its  freedom  from  disturbance.  The  following  items  are 
gathered  from  a  study  of  the  whole  series. 

a  Holes  6,  10,  12,  13,  15,  17,  18,  21,  22  and  25  penetrate  Helder- 
berg  limestone,  a  total  combined  depth  of  1096  feet.  Individual 
holes  vary  in  core  saving  from  39.3;/  (no.  13)  to  95.3$  (no.  15). 
The  average  core  saving  is  78.19$. 

b  Holes  8  and  9  are  in  Onondaga  limestone  with  a  total  pene- 
tration of  197  feet.  The  core  saving  varies  from  56.2^  to  92.8%' 
with  an  average  of  74.5^. 

c  Holes  11,  19,  20,  23,  24,  27  penetrate  Hudson  River  shale  and 
together  represent  a  total  of  696.5  feet.  The  core  saving  varies 
from  16.6$  to  89^,  with  an  average  of  42.1$. 

d  Holes  6,  10,  11,  12,  14,  16  and  20  cut  High  Falls  shales  to  a 
combined  total  of  410  feet.  The  saving  varies  in  different  holes 
from  17$  to  83.3^,  with  an  average  core  saving  of  44.5$. 

e  Holes  8  and  26  penetrate  Esopus  shale  and  penetrate  76  feet. 


142 


NEW  YORK  STATE  MUSEUM 


The  core  saving  varies  from  73$  to  84.6^,  making  an  average  of 
78.8;£. 

/  Holes  10,  11,  12,  14,  16,  19,  20,  23,  24  and  27  penetrate  Shawan- 
gunk  conglomerate  a  total  of  1356.5  feet.  Core  saving  varies  in 
different  holes  from  33.3$  to  100$.   The  average  recovery  is  60.52^'. 

y  Holes  6,  10,  12,  15  and  16  cut  Binnewater  sandstone.  The 
total  penetration  is  205  feet.  The  range  of  core  saving  is  from 
30.6^  to  74.7^,  with  an  average  of  56$. 

h  Holes  7  and  9  cut  Hamilton  shales  to  a  total  amount  of  65  feet. 
The  range  of  saving  is  70$  to  81. 8$,  with  an  average  of  75.9$. 

3  Artesian  flows.  Several  of  the  borings  struck  artesian  flow 
of  water.  The  fact  that  the  sources  of  this  flow  are  not  the  same 
has  led  to  a  tabulation  of  these  data. 


RECORD  OF  ARTESIAN  FLOWS 
Flow 

,'  Static  Flow  encountered 

Hole  Size  in      head  gallons       at  elevation 

no.     inches     in  feet     Minute     Day  Feet  Strata 

10  i  18         30     43  200     — 109  ....Binnewater  sandstone 

11  1  10  10     14400     — 60  .  . . .  Shawangunk  conglomerate 

12  y%        1    ■ — 24  ....High  Falls  shales 

14  Wa    +90  

20        Y&  7-5       i°     14400  +108  ...  .Shawangunk  conglomerate 

23  2    —    5  . . .  . 

31  2    +158  .... 

39  1J/2    +112  . . .  .Helderberg  limestone 

S'NE        Y%  12.4    ....  432  +203   Hamilton    shale  (possibly 

drift) 


Pumping  experiments  and  porosity  tests 

Systematic  tests  have  been  made  for  flow  of  water,  behavior  of 
ground  water  and  porosity  of  rock  on  certain  of  the  Rondout  ex- 
ploratory holes  under  the  direction  of  Mr  L.  White,  division  engi- 
neer. A  summary  of  these  tests  has  been  furnished  by  him  from 
which  is  quoted  the  following: 

In  addition  to  determining  the  location  and  thickness  of  the  beds 
and  the  general  character  and  condition  of  the  rock  from  inspection 
of  the  cores,  serious  attempts  were  made  to  determine  the  relative 
porosity  and  water-bearing  quality  of  the  rocks  encountered  for 
the  following  reasons.  (1)  To  determine  the  probable  leakage  from 
the  siphon  when  in  operation.  (2)  To  determine  the  probable 
amount  of  water  to  be  handled  in  construction.  These  experiments 
were  divided  into  three  classes:  (1)  Observation  of  flow  from  cer- 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


143 


tain  drill  holes  which  showed  sustained  flow  of  water.  (2)  Pres- 
sure tests  in  which  water  was  pumped  into  holes  which  had  been 
sealed  off  and  pressure  and  leakage  noted.  (3)  Pumping  tests  in 
which  water  was  pumped  from  4  inch  drill  holes  by  means  of  deep 
well  pump  of  the  type  used  in  oil  wells,  and  fall  of  ground  water 
during  pumping  and  subsequent  rise  after  cessation  of  pumping 
noted.  A  description  of  the  first  two  and  the  results  obtained  from 
them  follows : 

A  substantial  flow  of  water  was  observed  from  the  following 
holes : 

11/17:  50  gallons  per  minute  through  2^/2  inch  pipe,  static  head 
10  feet 

10/17:  30  gallons  per  minute  through  V/z  inch  pipe,  static  head 
18  feet 

20/17:  10  gallons  per  minute  through  }i  inch  pipe,  static  head 
7.5  feet 

The  static  head  was  observed  by  adding  on  lengths  of  pipe  until 
the  water  ceased  to  flow  over.  It  will  be  noticed  in  the  case 
of  hole  no.  10  that  the  flow  from  the  iJ/>  inch  pipe  is  not  that  due 
to  static  head  of  18  feet,  but  that  due  to  a  head  of  only  y2  foot.  In 
other  words  the  friction  head  is  about  17.5  feet,  and  the  velocity 
head  only  ^2  foot.  This' same  condition  holds  true  of  the  other 
holes  from  which  a  flow  was  obtained.  This  would  seem  to  indicate 
that  the  amount  of  water  is  not  very  great  but  that  it  is  under  con- 
siderable pressure.  It  is  believed  that  this  pressure  is  caused  by 
gas. 

A  slight  flow  was  observed  from  the  following  holes:  12/17, 
H/17,  23/17,  31/44,  39/22,  and  5/NE. 

The  flow  from  most  of  these  holes  has  ceased  since  the  pipe  used 
in  boring  was  withdrawn.  There  is  still  some  flow  from  the  follow- 
ing holes:  11/17,  20/17,  25/17  and  5/NE. 

The  flow  from  hole  11/17  is  constant  at  about  10  gallons  per 
minute.  The  others  are  too  small  to  be  measured.  It  will  be  noted 
that  the  only  substantial  flows  encountered  were  from  the  High 
Falls  shale,  Binnewater  sandstone  and  Shawangunk  grit,  and  that 
it  was  possible  to  force  water  into  these  rocks  in  greater  quantities 
and  at  a  less  pressure  than  in  the  other  shales  and  limestones. 

Porosity  tests.  The  method  of  making  these  tests  was  as 
follows : 

Wash  pipe  ecmipped  with  a  device  for  sealing  the  hole  was 
lowered  to  the  desired  elevation.  The  seal  consisted  of  alternate 
layers  of  rubber  and  wood  around  the  pir>e  preventing  the  water 
from  escaping  between  the  walls  of  the  hole  and  the  pipe.  Water 
was  then  pumped  in  and  pressure  and  leakage  noted. 

The  result  of  the  pressure  tests  was  to  show  in  a  general  way: 
( 1)  That  the  pressure  increased  with  the  depth  of  seal.  (2)  Thai 
the  leakage  decreased  with  the  depth  of  seal.  (3)  The  maximum 
pressure  in  the  grit  was  T40  pounds  to  the  square  inch  and  minimum 


•44 


NEW  YORK  STATE  MUSEUM 


leakage  was  5  gallons  per  minute.  (4)  In  the  Hamilton  shales  a 
pressure  of  300  pounds  to  the  square  inch  with  very  little  leakage 
was  obtained. 

The  unknown  factors  are  too  many  and  too  great  to  make  any 
reliable  deductions  from  these  experiments. 


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Fig. .23    Curve 'showing  fall  of  ground  water  level  while  pumping_from  boring  34 


Pumping  experiments  were  carried  on  in  holes  32/22  as  follows: 
The  apparatus  used  was  a  deep  well  pump  of  the  type  used  in  oil 
wells.  The  holes  were  of  an  inside  diameter  of  4*4  inches  and 
were  cased  to  the  bottom.  A  3/2  inch  working  barrel  was  then 
lowered  to  the  bottom  of  a  line  of  wooden  sucker  rods.  The  stroke 
was  44  inches  and  the  nominal  capacity  of  pump  at  38  strokes  per 
minute  was  60  gallons  per  minute  or  86,400  gallons  per  day.  The 
power  was  obtained  from  a  40  horsepower  boiler  and  35  horse- 
power engine  belted  to  a  10  foot  band  wheel  which  was  connected 
to  a  26  foot  walking  beam.  In  hole  32/22  at  station  607  +  50  the 
average  discharge  at  38  strokes  per  minute  was  90  gallons  per 
minute  or  129,600  per  day.  The  experiment  was  continued  for  15 
days  and  the  total  amount  of  water  pumped  was  1,071,000  gallons. 
The  ground  water  level  was  not  lowered.  It  will  be  noticed  that 
the  discharge  at  this  point  was  50$  in  excess  of  the  theoretical 
capacity  of  the  pump.  This  was  caused  by  the  presence  of  gas,  the 
effect  of  which  seemed  to  be  increased  by  the  churning  action  of 
the  pump.  This  may  also  explain  the  failure  to  lower  the  ground 
water. 

The  experiment  at  hole  34/22  was  similar  in  character.  The 
upper  230  feet  of  this  hole  had  an  interior  diameter  of  4] '4  inches 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


145 


and  the  bottom  274  feet  a  diameter  of  only  3^4  inches.  At  first  a 
2^4  inch  working  barrel  was  used  to  pump  from  the  bottom  and  a 
discharge  at  32  strokes  per  minute  averaged  24  gallons  per  minute 
or  34,500  gallons  per  day.  This  was  continued  for  about  15  days 
and  the  total  quantity  pumped  was  490,000  gallons.  The  ground 
water  level  was  lowered  17  feet  at  hole  34  and  4  feet  at  hole  32, 
750  feet  away. 

The  3/4  inch  pump  was  then  let  down  to  a  depth  of  200  feet 
with  a  2*/2  inch  casing  reaching  down  to  the  Binnewater  sandstone, 
depth  of  437  feet.  The  average  discharge  at  about  40  strokes  per 
minute  was  60-65  gallons  per  minute,  or  an  average  of  90,000  gal- 
lons per  day.  It  will  be  noted  that  the  discharge  was  much  smaller 
than  at  hole  32  owing  to  the  absence  of  gas.  Pumping  with  a 
3/4  inch  pump  was  continued  16  days  and  1,532,000  gallons  of 


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Fig.  24    Diagram  showing  successive  stages  of  ground  water  level  between  holes  32  and 

34  during  pumping 


water  were  pumped  in  addition  to  the  490,000  gallons  from  the  2^4 
inch  pump.  The  ground  water  level  in  hole  34  was  lowered  36  feet 
in  addition  to  the  17  feet  by  the  2  %  inch  pump,  but  rose  9  feet  in 
20  minutes,  and  30.5  feet  in  the  next  five  days.  In  the  next  22  days 
it  rose  9.15  feet,  or  .42  feet  per  day. 

Reduced  water  level  in  hole  32,  750  feet  away  by  pumping  in  34, 
15  feet,  or  1  foot  for  each  120,000  gallons  pumped,    In  the  first 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


M7 


three  clays  after  pumping  ceased  water  rose  5.2  feet,  and  in  22  days 
rose  9.8  feet  or  at  the  rate  of  0.45  feet  per  day. 

During  construction1  shaft  4  located  at  same  point  as  hole  32/22, 
station  607  +  50,  has  proved  a  very  wet  shaft,  the  inflow  varying 
from  400  to  850  gallons  per  minute.  Pumping  at  this  shaft  has 
lowered  the  general  water  level  and  correspondingly  lowered  the 
water  level  in  hole  34/22  at  station  600  +  00. 


Fig.  26     Curve  showing  rise  of  water  in  holes  32  and  34  after  pumping  ceased  in  hole  34 


1  From  this  shaft  after  reaching  tunnel  grade, —  250  feet,  and  after  running 
northward  into  the  fault  zone  and  porous  shales,  the  contractors  are  pumping 
1300  gallons  per  minute. 


CHAPTER  VII 


THE  WALLKILL  VALLEY  SECTION 

Between  the  Rondout  and  Wallkill  valleys  the  aqueduct  is  to 
follow  a  tunnel  at  hydraulic  grade  which  so  far  as  can  be  seen  will 
cut  only  Shawangunk  conglomerate  and  Hudson  River  slates.  No 
doubt  there  are  many  complicated  small  structures  which  because 
of  the  nature  of  the  slates  can  not  be  reconstructed.  The  work 
of  tunneling  is  not  advanced  far  enough  to  add  anything.  But 
in  the  Wallkill  valley,  where  it  is  necessary  again  to  plan  a  pressure 
tunnel  several  hundred  feet  below  grade,  a  considerable  amount 
of  exploration  has  been  carried  on.1 

These  explorations  [see  sketch  map  fig.  8]  are  distributed  along 
several  lines  crossing  the  valley  at  intervals  between  Springtown, 
about  3  miles  north  of  New  Paltz,  and  Libertyville,  which  is  about 
an  equal  distance  south. 

The  geology  is  simple.  Only  Hudson  River  slates  form  the  rock 
floor,  and  so  far  as  can  be  judged  no  other  formation  is  likely  to 
be  cut  by  the  tunnels.  There  are  no  doubt  many  complicated  struc- 
tures, both  folds  and  faults,  as  indicated  by  the  high  dips,  but  again 
because  of  the  nature  of  this  rock  it  is  impossible  to  discriminate 
closely  enough  between  different  beds  to  determine  exact  relations. 
The  point  of  greatest  practical  importance  lies  in  the  fact  that 
the  rock  is  fairly  uniform  and,  although  much  disturbed  is  of 
such  nature  that  crevices  and  joints  or  fault  zones  are  almost  as 
impervious  as  the  undisturbed  rock.  This  is  because  of  the  tend- 
ency of  a  formation  of  this  composition  to  heal  itself  with  fine, 
compact  clay  gouge.  In  fact,  the  mechanical  disturbance  produces 
or  develops  the  cement  filling  contemporaneously  with  the  move- 
ment. It  is  chiefly  a  mechanical  filling,  whereas  the  healing  of  a 
harder  and  more  brittle  rock  like  a  granite  or  a  limestone  requires 
more  chemical  assistance. 

An  additional  practical  question  involves  the  estimate  of  depth 
required  to  avoid  any  possible  buried  Prepleistocene  gorges  and 
maintain  a  safe  cover  to  guard  against  undue  leakage  or  rupture. 

1  Explorations  on  the  Wallkill  division  are  carried  on  under  the  direction 
of  Lawrence  C.  Brink,  division  engineer.  The  final  construction  is  in  charge 
of  James  F.  Sanborn,  division  engineer,  with  headquarters  at  New 
Paltz,  N.  Y. 

149 


NEW   YORK  STATE  MUSEUM 


To  this  end  most  of  the  explorations  were  made.  Two  lines  less 
than  a  mile  apart  on  which  a  few  exploratory  borings  were  made 
near  Springtown  indicate  two  buried  channels,  a  master  channel 
and  a  tributary  from  the  west  which  converge  northward.  A 
maximum  depth  reaching  70  feet  below  sea  level  was  found  on  the 
more  northerly  line  almost  directly  beneath  the  present  stream 
channel  which  flows  on  drift  at  an  elevation  of  150  above  tide. 

The  more  southerly  profile  reaches  only  sea  level  indicating  a 
gradient  for  the  preglacial  stream  at  this  immediate  locality  of  more 
than  79  feet  per  mile. 

In  the  vicinity  of  Libertyville,  5  to  6  miles  farther  south,  where 
the  aqueduct  was  finally  located,  the  profile  was  found  to  be  con-, 
siderably  higher.  Intermediate  profiles  are  shown  in  accompany- 
ing figures.  The  deepest  point  yet  found  on  the  Libertyville  line 
is  65  feet  above  sea  level.  It  is  worth  noting  that  the  gradient  of 
the  ancient  Wallkill  is  therefore  shown  to  be  decidedly  unsymmet- 
rical.  The  rock  floor  formation  remains  the  same  although  it  may 
vary  somewhat  in  character.  Under  these  circumstances,  however, 
a  gradient  of  13  feet  per  mile  from  Libertyville  to  Springtown 
forms  a  sharp  contrast  with  the  79  feet  per  mile  represented  at 
the  Springtown  locality.  In  view  of  the  remarkable  increase  of 
gradient  and  the  narrower  form  it  seems  reasonable  to  regard  this 
as  a  rejuvenation  feature  developed  at  the  time  of  extreme  con- 
tinental elevation. 

How  much  deeper  the  lower  Wallkill  may  be,  including  the  so 
called  Rondout  river,  which  is  really  a  continuation  of  the  ancient 
Wallkill  and  geologically  belongs  to  this  drainage  line  instead  of 
to  the  Rondout,  no  one  can  tell.  But  it  is  at  least  interesting  to 
observe  that  the  intervening  distance  from  Springtown  to  the  Hud- 
son at  Kingston  is  approximately  12  miles  and  that  a  gradient  for 
that  distance  equal  to  the  average  known  in  the  6  miles  explored, 
i.  e.  24  feet  per  mile,  would  depress  the  outlet  288  feet  more. 
That  would  be  equivalent  to  367  feet  below  sea  level.  If,  how- 
ever, a  steep  gradient  such  as  that  at  Springtown  prevails  in  this 
lower  portion  it  is  necessarily  much  lower  —  for  example  if  a  79 
foot  gradient  is  maintained  it  would  be  possible  to  reach  a  final 
outlet  at  — 1029  feet.  It  is  likely  that  an  intermediate  value  is 
more  nearly  correct.  This  has,  however,  an  important  bearing 
upon  the  question  of  maximum  Hudson  river  depth,  especially  the 
existence  of  an  inner  deeper  gorge  above  the  Highlands.  So  far 
as  this  Wallkill  profile  goes,  it  supports  the  gorge  theory.  It  is 
certain  that  the  Prepleistocene  Wallkill  flowed  north  not  very  dif- 


Plate  25 


NO.HQUE.DEPT.  B.W.S.         N.Y.C.  WALLK'LLDIV. 

PROFILE  SHOWING  WASH  AND  COffC  BORINGS 
LINE  A  -  SPRINGTOWN 


0; 


,'.■/!  L  L     I L  L      5 1PHON 


Cross  section  showing  the  buried  preglacial  Wallkill  channel  as  indi- 
cated by  exploratory  borings  near  Springtown 


WALLKILL  SIPHON 


Profiles  of  the  present  and  preglacial  Wallkill  channels  near  Liberty- 
ville,  and  a  diagrammatic  section  showing  the  different  types  of  drift-filling 
together  with  the  borings  which  furnished  the  data 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  1 5 1 

ferently  from  the  present  stream  except  on  a  steeper  gradient,  but 
in  all  probability  the  headwater  supplies  between  this  stream  and 
the  Moodna  have  been  somewhat  shifted.  It  is  possible  that  some 
former  Moodna  drainage  area  is  now  tributary  to  the  Wallkill. 
But  these  changes  were  wholly  glacial  in  origin  and  the  extent  of 
such  shift  is  indeterminate  at  present. 

It  is  a  notable  fact  that  a  large  proportion  of  the  work  of  ex- 
ploration in  this  valley  was  done  successfully  by  the  wash  rig. 

The  extensive  lot  of  data  was  gathered  without  much  delay  or 
difficulty.  This  is  because  of  the  nature  and  origin  of  the  drift 
cover.  A  considerable  proportion  of  the  drift  mantle  especially 
in  central  and  deeper  portion  of  the  valley  is  modified  assorted 
sands,  gravels  and  silts  or  muds.  In  part  they  represent  deposits 
in  standing  water  laid  down  at  a  time  when  the  lower  (north)  end 
of  the  valley  was  obstructed  by  ice  and  while  waste  was  poured 
into  the  valley  from  neighboring  ice  fields.  It  is  impossible  to 
reconstruct  the  beds  of  these  materials  with  any  degree  of  accuracy. 
But  it  is  at  least  certain  that  lens  or  wedgelike  layers  of  differ- 
ent quality  of  material  were  penetrated,  indicating  oscillation  and 
overlapping  of  deposition  conditions,  boulder  beds  and  till  being 
interlocked  with  assorted  sands  and  gravels.  But  there  is  appar- 
ently no  evidence  of  ice  deposits  of  greatly  differing  age.  The 
accompanying  profile  and  cross  section  is  a  representation  of  ma- 
terials on  the  Libertyville  line  based  upon  identifications  made  by 
the  inspector  of  the  Board  of  Water  Supply  of  the  Wallkill  Divi- 
sion under  Mr  L.  C.  Brink,  division  engineer. 


CHAPTER  I'lII 


ANCIENT  MOODNA  VALLEY 

Moodna  creek  enters  the  Hudson  from  the  west  between  Corn- 
wall and  Newburgh  not  more  than  a  mile  north  of  the  entrance 
to  the  Highlands.  It  is  a  retrograde  stream  in  its  backward  flow 
similar  to  the  Wallkill.  But  its  channel  at  present  is  almost 
wholly  on  glacial  drift  which  it  has  trenched  to  a  depth  of  more 
than  100  feet  below  the  average  adjacent  surface.  How  much 
of  its  retrograde  course  therefore  may  be  postglacial  is  not  so 
clear.  It  seems  necessary,  however,  to  account  for  all  drainage 
on  the  north  margin  of  the  Highlands  by  streams  flowing  to  the 
Hudson  northward.  There  is  no  notch  low  enough  for  their  escape 
elsewhere.  The  ancient  Moodna  must  have  carried  most  of  this 
run-off  from  the  district  occupying  the  angle  between  the  Wall- 
kill  and  the  Highlands.  This  stream  ma}'  have  drained  even  more 
of  the  region  now  forming  the  divide  with  the  Wallkill  than  does 
the  present  Moodna.  In  any  case  it  must  have  been  a  stream  of 
considerable  size,  capable  of  excavating  a  valley  or  gorge  of 
greater  prominence  during  the  period  of  early  Pleistocene  rejuvena- 
tion than  now  appears.  Furthermore  its  position  makes  it  highly 
probable  that  tributaries  of  fair  size  entering  in  its  lower  course 
were  also  effective  enough  to  require  consideration.  This  conclu- 
sion has  led  to  the  exploration  of  the  Moodna  valley  in  consider- 
able detail  in  preparation  for  the  aqueduct  work. 

The  Catskill  aqueduct  is  to  cross  the  stream  near  Firth  Cliffe, 
which  lies  almost  directly  west  of  Cornwall -an-Hudson,  and  be- 
cause of  the  low  surface  elevation  across  this  valley,  as  in  the 
others,  a  pressure  tunnel  in  rock  is  judged  to  be  the  most  suitable 
type  of  structure.  The  accompanying  sketch  map  shows  the 
location. 

Explorations  were  conducted  especially  for  the  buried  channels 
and  character  of  rock  floor. 

Geologic  features 

The  region  is  one  of  chiefly  Hudson  River  slate.  But  there 
are  inliers  of  the  older  rocks  such  as  Snake  hill  which  belongs  to 
a  long  ridge  of  Precambric  gneiss  and  granite,  brought  to  the  sur- 
face by  folding  and  faulting  and  there  are  more  rarely  outliers  of 
younger  formations  such  as  Skunnemunk  mountain.    Farther  north 

[53 


154 


NEW  YORK  STATE  MUSEUM 


at  Newburgh  a  gneiss  ridge  is  accompanied  by  limestone,  but  in  its 
soutberly  extension  the  slates  are  in  direct  contact.  This  relation 
is  believed  to  be  wholly  due  to  faulting  on  both  limbs  of  the  anti- 
clines. This  gneiss  ridge  disappears  southward  beneath  the  drift, 
but  the  borings  have  shown  that  it  continues  across  the  aqueduct 
line,  although  it  has  lost  its  influence  on  the  topography.'  There 
are  other  inliers  of  similar  character  such  as  Cronomer  hill  3  miles 
northwest  of  Newburgh.  Between  these  two  gneiss  ridges  lies  the 
southerly  extension  of  the  Wappinger  limestone  belt.  But  so  far 
as  is  known  it  disappears  beneath  the  Hudson  River  series  long 
before  reaching  the  line  of  exploration. 

Near  Idlewild  station,  filling  the  space  between  the  two  branches 
of  the  Erie  Railroad,  there  is  a  syncline  containing  the  series  of 
Siluric  and  Devonic  strata  which  spreads  southwestward  to  include 
Skunnemunk  mountain,  an  outlier  of  Devonic  strata.  This  is  the 
only  occurrence  of  these  formations  in  this  region  south  of  the 
Rondout  valley.  The  structure  and  stratigraphic  features  of  this 
occurrence  have  been  worked  out  by  Hartnagel.  Its  northward 
extension  in  all  probability  terminates  abruptly  by  a  cross  fault  not 
far  north  of  the  Ontario  and  Western  Railroad. 

From  these  occurrences  southward  to  tine  Highlands  proper 
nearly  everything  to  be  seen  through  the  drift  is  Hudson  River 
slates. 

The  Highland  gneisses  are  bounded  on  the  north  side  by  a  fault 
or  series  of  faults.  This  brings  various  members  of  the  overlying 
series  into  contact  along  the  margin.  In  the  best  place  where  a 
direct  observation  can  be  made  the  gneisses  are  thrust  over  upon 
the  Hudson  River  slates  along  a  plane  that  dips  about  40  degrees 
to  the  northeast.  It  is  probable  that  a  displacement  of  as  much 
as  2000  feet  or  more  could  reasonably  be  assumed  at  this  place. 
The  contact  zone  also  is  much  crushed  and  bears  every  evidence 
of  having  undergone  extensive  disturbance  of  this  kind.  Others 
of  this  same  type  occur  within  the  gneisses  where  weaknesses 
formed  in  this  way  permit  the  development  of  such  notches  as 
Pagenstechers  gorge.  In  some  cases  the  rock  beneath  the  surface  in 
these  zones  is  more  decayed  and  less  substantial  than  that  at  the 
surface. 

Exploration 

The  first  borings  made  with  the  wash  rig  were  found  extremely 
unreliable  in  the  Moodna  valley.  That  is  because  of  the  very 
heavy  bouldery  drift  forming  the  greater  part  of  the  filling  on  the 
ancient  topography.    Next  to  the  Hudson  river  gorge  itself,  no 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


155 


place  has  presented  greater  difficulties  in  penetrating  this  drift  man- 
tle. Boulders  of  such  immense  size  occur  that  they  have  to  be 
drilled  like  bed  rock.  In  one  of  the  holes  a  boulder  30  feet 
through  was  penetrated  and  100  feet  more  of  drift  found  below. 
Progress  in  such  ground  is  extremely  slow  and  costly.  This  is 
so  much  the  more  so  where  as  in  this  case  there  are  long  stretches 
with  unusually  deep  cover. 

A  glance  at  the  accompanying  profile  and  cross  section  will  show 
a  very  deep  and  wide  valley.  Many  of  the  borings  are  more  than 
300  feet  in  drift  which  almost  wholly  obscures  the  ancient  topog- 
raphy. The  present  Moodna  is  about  half  as  deep  and  occupies 
the  extreme  eastern  margin  of  the  older  gorge.  There  is  a  sec- 
ondary gorge  on  the  west  separated  from  the  main  channel  by  a 
sharp  divide.  A  few  other  smaller  notches  in  the  line  represent 
smaller  tributary  or  independent  stream  courses.  One  of  these 
of  much  interest  is  known  as  Pagenstechers  gorge. 

The  rock  floor  at  all  points  except  two  in  the  central  Moodna 
valley  including  its  two  nearest  tributaries  is  Hudson  River  shales, 
slates  and  sandstones  of  considerable  variation,  sometimes  much 
brecciated.  The  two  exceptional  borings  are  no.  8/A44  and  no. 
16/A44  on  the  west  flank  of  the  westerly  tributary  gorge,  and 
they  are  in  pegmatite  and  granitic  gneiss  which  is  in  all  probability 
the  narrow  southerly  extension  of  the  Snake  hill  ridge.  Here 
again  neither  quartzite  nor  limestone  were  found  on  the  flank,  a 
condition  that  seems  to  support  the  view  of  a  double  fault  along 
the  Snake  hill  ridge. 

In  striking  contrast  with  the  broad  central  Moodna  are  the  two 
narrow  and  very  deep  notches  farther  to  the  east,  the  first  in 
slates  and  the  second  (Pagenstechers)  in  Highlands  gneiss. 

Special  features 
Course  of  the  Moodna.    The  chief  interest  centers  around  the 
Moodna   channel.     There   are   several   unusual   conditions,  for 
example : 

The  rock  floor  along  the  profile  is  almost  flat  for  a  distance  of 
nearly  half  a  mile  in  spite  of  the  fact  that  there  would  seem  to 
be  every  reason  for  a  different  form.  The  differences  in  hard- 
ness of  rock  floor  alone  would  encourage  differential  erosion ;  and, 
since  the  structure  of  the  formations,  the  strike,  is  almost  parallel 
to  the  supposed  course  of  the  stream,  the  influence  of  different 
beds  would  be  at  a  maximum.  Furthermore,  the  deep  gorge  of 
the  Hudson,  into  which  the  stream  flowed  is  only  2  miles  away; 


156 


NEW   YORK  STATE  MUSEUM 


and  if  that  gorge  represents  stream  erosion  to  such  depth  (over  750 
feet)  it  would  indicate  a  gradient  of  nearly  300  feet  to  the  mile 
for  the  last  2  miles  of  the  Moodna  —  a  condition  to  say  the  least 
decidedly  unfavorable  to  the  development  of  a  flat-bottomed  valley. 

Of  course,  if  the  profile  as  determined  can  be  assumed  to  run 
exactly  parallel  to  the  old  stream  channel  for  half  a  mile  it  would 
be  less  surprising.  But  even  then  it  is  too  flat.  For  so  short  a  dis- 
tance from  the  Hudson  gorge  the  gradient  ought  to  be  much 
greater  than  the  variation  observed  in  the  Moodna  channel.  There 
are  certainly  reasons  in  the  structural  geology  favoring  a  northeast 
course  instead  of  one  parallel  to  the  profile  line.  And  if  the 
stream  really  did  flow  across  this  structure,  the  differences  of 
hardness  of  beds  ought  to  have  encouraged  a  much  greater  differ- 
ence in  depth  of  channel  than  the  profile  presents.  With  structures 
all  running  northeast  there  is  every  reason  to  expect  the  stream 
to  follow  them. 

Recent  exploratory  data  strongly  supports  the  theory  that  the 
Hudson  gorge  at  Storm  King  gap  is  widened  and  possibly  some- 
what overdeepened  by  glacial  ice.  Under  normal  stream  relations 
one  might  consider  the  Moodna  a  tributary  hanging  valley,  itself 
rounded  and  smoothed  to  a  broad  U-shape  by  ice.  This  would  be 
a  very  easy  solution  if  it  were  not  for  the  fact  that  this  tributary 
Moodna  opens  into  the  Hudson  as  a  reversed  stream,  i.  e.  it  opens 
against  the  flow  of  the  Hudson  and  more  or  less  directly  against 
the  known  ice  movement.  It  can  not  be  a  hanging  valley  there- 
fore of  the  normal  sort.  If  a  hanging  valley  of  ice  origin  at  all 
it  would  necessarily  be  one  therefore  gouged  out  by  ice  moving 
from  its  mouth  toward  its  head,  a  case  that  so  far  as  the  writer 
knows  has  never  been  observed.  The  chief  objection  to  this  theory 
is  that  in  no  other  gorge  or  channel  (with  one  exception,  the  Hud- 
son at  Storm  King  gap)  anywhere  in  the  region  so  far  as  known 
is  there  any  evidence  of  serious  modification  of  an  original  stream 
channel  by  the  ice  invasion.  Of  course,  the  axis  of  the  valley  is 
favorable  and  the  situation  is  peculiar  in  that  it  parallels  the  High- 
lands front  in  this  vicinity  and  the  action  of  the  ice  may  be  as- 
sumed to  have  been  somewhat  concentrated  along  this  margin  be- 
cause of  the  obstruction. 

Inner  notch  or  secondary  gorge.  Those  who  habitually  em- 
phasize ice  action  would  no  doubt  choose  to  regard  this  whole  val- 
ley as  shown  in  the  profile,  as  chiefly  glacial  in  character  and 
origin.  Tf  that  explanation  is  the  true  one,  then  it  must  be  ad- 
mitted that  a  deeper  smaller  inner  notch  or  gorge  is  unnecessary 
and  indeed  unlikely. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


157 


The  critical  point  therefore  in  the  whole  argument  is  as  to  the 
origin  of  the  Yalley,  i.  e.  is  it  essentially  a  stream  valley  ?  Or  is  it 
as  to  present  rock  floor  form  wholly  a  glacial  valley  ?  . 

If  it  is  a  stream  valley  then  no  doubt  full  account  must  be  taken 
of  the  proximity  to  the  Hudson,  and  the  possibility  of  developing 
a  temporary  graded  condition  and  some  adequate  allowance  must 
be  made  for  its  work  during  the  subsequent  continental  elevation 
and  the  deepening  of  that  river  to  several  hundred  feet  below  the 
known  bottom  of  the  Moodna.  In  short,  one  would  expect  a  nar- 
row deeper  notch  in  the  Moodna  floor  as  a  result  of  this  rejuvena- 
tion. But  on  the  contrary  if  in  preglacial  time  the  stream  were 
not  so  powerful  and  had  not  been  able  to  keep  pace,  and  if  the 
ice  movement  can  be  assumed  to  have  concentrated  along  this  line 
to  such  efficiency  as  to  gouge  out  a  groove  3000  feet  wide  almost 
flat  to  a  depth  of  300  feet  only  guided  in  direction  by  the  original 
Moodna,  then  one  may  readily  abandon  the  idea  of  a  deeper  notch. 

One  or  the  other  of  these  types  of  origin  must  be  the  chief 
factor  in  reaching  a  reasonable  opinion  as  to  the  presence  of  an 
inner  notch. 

In  any  attempt  to  choose  between  these  factors,  one  is  led  -to 
reconstruct  the  preglacial  drainage  lines.  When  this  is  done  it  at 
once  appears  as  most  probable  that  there  was  at  that  time  as  now 
a  considerable  area  tributary  to  the  Hudson  with  a  stream  course 
very  much  like  the  present  Moodna.  In  other  words  a  fair  sized 
stream  is  assured.  Once  such  a  stream  is  granted  and  the  effects 
of  its  work  reckoned  in  full  knowledge  of  the  adjacent  Hudson, 
and  its  probable  behavior  is  studied  in  the  light  of  the  data  ob- 
tained in  exploration  of  the  valleys  of  other  tributaries,  it  becomes 
more  and  more  difficult  to  wholly  eliminate  the  inner  gorge  idea. 
It  seems  to  the  writer  probable  that  the  valley  owes  its  erosion 
chiefly  to  the  preglacial  stream.  But  the  channel  has  suffered  sub- 
sequent widening  and  smoothing  by  ice  especially  in  its  upper  and 
broader  portion,  below  which  there  may  yet  be  a  notch.  One  must 
admit  that  the  results  of  boring  prove  the  notch  to  he  very  nar- 
row, less  than  150  feet,  or  else  not  there  at  all.  In  reaching  an 
opinion  as  to  the  possibility  of  one  so  narrow,  it  is  worth  while 
to  note  that  the  Esopus,  which  is  a  larger  stream,  has  cut  down 
at  Cathedral  gorge  to  a  depth  of  from  50  to  80  feet  with  almost 
vertical  sides  and  only  about  150  feet  wide.  This  gorge  further- 
more is  cut  in  almost  horizontal  strata  of  such  character  that 
there  is  no  special  structural  tendency  in  them  to  contract  the 
stream.    At  the  Moodna  on  the  contrary,  in  addition  to  the  smaller 


158 


NEW  YORK  STATE  MUSEUM 


size  of  stream,  the  rocks  stand  on  edge  and  run  parallel  to  the 
supposed  course  so  that  this  structural  influence  is  toward  a  nar- 
row and  reasonably  straight  gorgelike  form.  It  is  not  only  pos- 
sible that  the  gorge  is  narrow,  but  even  probable  that  it  is  narrower 
than  the  present  Moodna,  i.  e.  less  than  100  feet  wide. 

How  deep  such  an  inner  gorge  may  be  if  it  does  exist  is  a  prac- 
tical question  in  this  particular  case,  because  its  depth  has  a  direct 
influence  on  choice  of  depth  of  pressure  tunnel.  Because  of  the 
evident  narrowness  it  is  likely  that  it  is  not  of  very  great  depth 
—  in  view  of  the  quality  of  these  shales  perhaps  not  over  a  hun- 
dred feet. 

Is  there  any  one  point  more  than  another  favorable  for  such  a 
notch?  There  are  two  facts  bearing  on  this  question,  (i)  the  vari- 
ation in  core  saving  which  indicates  that  hole  no.  5/A44  with 
7<t  has  a  recovery  of  only  1/5  the  average,  and  (2)  the  fact  that 
hole  no.  15/A44-I-  ,  which  is  the  next  hole,  shows  the  lowest  bed 
rock  in  this  valley.  On  the  ground  of  profile  therefore  and  on  the 
ground  of  structural  weakness  there  is  reason  to  choose  this  space 
between  no.  5/A44  and  no.  15/A44  as  the  most  likely  position. 

Summary.  The  very  abnormal  profile  of  the  Moodna  valley 
based  upon  the  borings  may  be  due  either  (1)  to  parallelism  with 
the  stream  course,  or  (2)  to  a  graded  condition  of  the  stream  in 
some  preglacial  epoch,  or  (3)  to  modification  of  an  original  less 
prominent  channel  by  ice  erosion. 

It  is  the  opinion  of  the  writer  that  the  ancient  stream  crossed  the 
profile  line  much  as  the  present  stream  does,  that  the  additional 
narrower  valley  immediately  to  the  west  side  is  that  of  a  pre- 
glacial tributary  instead  of  a  bend  of  the  Moodna  itself,  that  there 
was  a  development  of  a  moderate  sized  somewhat  flattened  valley 
corresponding  to  the  benches  and  shelves  noted  in  other  streams, 
including  the  Hudson,  that  subsequent  elevation  of  the  continent 
rejuvenated  the  stream  which  cut  a  deeper  narrow  inner  notch,  that 
glacial  ice  moving  in  reverse  direction  widened  and  smoothed  this 
upper  portion  of  the  valley,  but  that  there  may  yet  be  a  remnant 
ot  the  deeper  notch  in  its  bottom,  and  that  the  space  between  holes 
no.  5/A44  and  no.  15/A44  is  the  most  likely  location  of  this  inner 
gc  rge. 

Tributary  divide.  The  sharp  divide  between  the  two  deep 
portions  of  the  valley  bottom  has  proven  an  evasive  feature  in  the 
later  exploration.  Two  holes  put  down  a  short  distance  to  the 
southward  (24/A44  and  20/A44)  failed  to  find  the  rock  floor  so 
high,  one  reaching  rock  at  a  depth  of  181  feet  and  the  other  failing 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


159 


to  find  rock  even  at  213  feet.  Two  others  nearly  a  thousand  feet 
to  the  westward,  however,  found  rock  again  at  approximately  the 
same  elevation  as  the  divide.  If  this  is  a  tributary  stream  divide 
therefore  it  must  have  an  east-west  trend. 

Pagenstechers  gorge 

This  is  a  notch  between  Storm  King  ridge  and  Little  Round  top 
occupied  by  a  very  small  mountain  stream.  The  rock  floor  is  granite 
gneiss  of  the  Storm  King  type.  Its  special  characters  are  (1) 
extreme  shattering  or  crushed  condition,  and  (2)  extensive  decay 
along  this  zone  which  has  softened  the  rock  constituents  to  great 
depth. 

Considering  the  nature  of  the  granite  gneiss  in  general  this  nar- 
row gorge  is  a  surprisingly  deep  one.  But  this  is  no  doubt  due  to 
the  influence  of  the  decayed  crush  zone.  The  drill  cores  taken  from 
the  holes  that  penetrated  the  floor  at  this  place  are  so  much  altered 
that,  after  several  months  exposure  to  the  air,  they  can  be  readily 
crushed  in  the  hand.  Hole  no.  16/A45  which  is  centrally  located 
penetrated  to  — 196  feet.  It  is  in  material  of  this  same  condition, 
to  at  least  — 100  feet.  Similar  conditions  are  proven  to  the  north 
of  the  line,  shown  in  the  accompanying  profile  and  a  rapid  increase 
in  depths.  From  the  surface  outcrops  farther  up  the  gulch  it  is  easy 
to  see  that  the  crushed  zone  extends  in  that  direction  with  the 
strongest  lines  about  s.  70  w.  This  is  doubtless  on  the  strike  of  the 
fault  lines  of  the  northern  border  of  the  range.  It  is  of  more  than 
usual  interest  in  showing  the  depth  to  which  incipient  decay  has 
penetrated  in  these  crush  zones,  and  the  efficiency  of  stream  erosion 
along  them. 

Overthrust  fault 

The  principal  fault  line  follows  the  margin  of  the  granite  gneisses. 
At  the  best  exposure  of  it  the  Hudson  River  slates  are  overridden 
by  the  gneiss.  This  represents  therefore  the  cutting  out  entirely  of 
the  Wappinger  limestone  and  the  Poughquag  quartzite  and  a  part 
of  the  slates  by  the  displacement  which  must  amount  to  at  least 
2000  feet  and  probably  more.  The  same  relation  is  indicated  by  the 
borings  and  by  the  outcrop  near  the  village  of  Cornwall,  but  a  little 
limestone  is  found  midway  between  the  two  points  along  the  strike 
of  the  fault.  The  strike  of  the  fault  averages  about  n.  650  to  70°  e., 
but  locally,  at  the  best  exposure,  it  is  only  n.  350  e.  The  dip  is 
southeast  at  an  angle  of  approximately  45  degrees. 


i6o 


NEW  YORK  STATE  MUSEUM 


Statistics 

Moodna  valley 

I    HOLES  BORED  UNDER  AGREEMENT  NO.  l8 


No. 
A18 

Surface 
elevation  in 
feet 

Rock 
elevation  in 
feet 

Rock 
penetration 
in  feet 

Per 

cent  core 
saved 

Kind  op  rock 

I 

86 

+  27 

22 

0 

Slate  and  sandstone 

(On  porosity  test  with  plug  at  58  feet  deep  the  loss  of  water  was  6  gallons 
per  minute  with  pressure  of  0-10  pounds  per  square  inch.)  Test  unsatis- 
factory because  of  large  hole. 


2      I    236.5  I  ?  o      I  o  1  o 

'    3  136-3  '         36- 7  '         26      I  o  I  Slate 

(On  porosity  test  with  depth  to  plug  173.5  feet  and  pressure  0-60  pounds 
per  square  inch  the  loss  was  5  gallons  per  minute.)  Test  unsatisfactory 
because  of  large  hole. 


u 

259 

6 

39 

4 

i54 

5 

75 

Slate  and  sandstone 

5 

295 

5 

37 

5 

129 

2 

7i 

Slate  and  sandstone 

6 

302 

7 

? 

0 

0 

0 

7 

297 

0 

+  26 

3° 

7 

0 

Slate 

2  HOLES  BORED  UNDER  AGREEMENT  NO.  40 


No. 
A40 

Surface 

Rock 

Rock 

Per 

elevation 

in 

elevation 

in 

penetration 

cent  core 

Kind  of  rock 

feet 

feet 

in  feet 

saved 1 

•Ti 

276 

201 

125 

0 

Slate 

2 

274 

3 

228 

8 

52 

5 

0 

3 

294 

7 

257 

7 

45 

3 

0 

'~  4 

273 

1 

222 

1 

139 

7 

60 

Slate  and  sandstone 

■:■  5 

347 

1 

239 

1 

48 

7 

O 

Slate 

6 

374 

6 

250 

6 

38. 

0 

0 

7 

168 

4 

40 

4 

1 10 

1 

88 

Slate  and  sandstone 

8 

188. 

7 

168 

2  • 

325 

76 

Slate  and  sandstone 

9 

1  76 

3 

164 

3 

25- 

5 

0 

Slate 

10 

172. 

3 

46 

3 

26 

0 

1 1 

169 . 

9 

98 

9 

25- 

5 

0 

1 2 

221 . 

2 

208 

2 

25- 

S 

0 

Slate  and  sandstone 

13 

226 . 

7 

210 

7 

32- 

7 

0 

14 

226 . 

6 

212 

6 

31  • 

0 

0 

1 5 

230 . 

1 

21S 

1 

32- 

5 

0 

Slate  and  sandstone 

16 

208. 

3 

184 

3 

32  • 

0 

0 

Slate 

17 

169 . 

2 

43 

2 

25- 

0 

0 

1  In  cases  which  show  no  recovery  of  core  a  method  of  drilling  was  employed  different 
from  the  others  and  the  rock  was  ground  to  pieces.  Failure  to  recover  core  may  therefore 
be  no  indication  of  poor  rock  quality. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  l6l 


3  HOLES  BORED  UNDER  AGREEMENT  NO.  44 


No. 
A44 

Surface 

rtOCK 

Reck 

rer 

elevation 

in 

elevation 

in 

penetration 

cent  core 

Kind  of  rock 

feet 

feet 

in  feet 

saved 

1l 

313 

6 

—  r7 

4 

167 

5 

*5 

Slate  and  sandstone 

l2 

2  74 

5 

+  171 

•  5 

229 

6 

20 

Slate  and  sandstone 

3 

282 

7 

+  39 

7 

58 

8 

14 

olate 

4 

277 

8 

— 22 

2 

166. 

7 

26 

Slate  and  sandstone 

5 

299 

5 

— 47 

0 

5°  • 

9 

7 

Slate  and  sandstone 

< 
0 

279 

5 

— 40 

3 

43- 

0 

27 

slate 

7 

299  . 

2 

— 5i 

6 

102  . 

2 

27 

Slate 

Q 
O 

277. 

+  09 

0 

109 . 

3 

57-6 

Granite    gneiss  and 
quartz 

9 

282  . 

6 

+  7 

6 

90. 

J3 

Slate  and  sandstone 

10 

230. 

5 

j 

IS4- 

7 

40 

Slate  and  sandstone 

1 1 

249. 

s 

—32 

5 

93- 

S 

I  I 

Slate  and  sandstone 

12 

272  . 

0 

—45 

58- 

6 

3° 

Slate  and  sandstone 

13 

288. 

4 

—37 

6 

79- 

0 

13 

Slate 

14 

185. 

1 

—39- 

9 

91 . 

8 

32 

Slate  and  sandstone 

15 

301  • 

8 

—59- 

2 

75- 

45 

Slate  and  sandstone 

16 

277. 

3 

+  25- 

9 

104 . 

6 

43 

Pegmatitic  granite 

17 

300  . 

— 42  ■ 

75- 

2 

33 

Slate  and  sandstone 

1  Porosity  test  made  on  hole  no.  1  shows  a  loss  of  .03  gallons  of  water  uider  100  pounds 
pressure  with  packer  at  depth  of  3S7  feet.    Depth  to  grjun  1  water  217  feet. 


Porosity  test  on  hole  2/A44. 

Ground  water  level  at  a  depth  of  90  feet  =  el.  +  184.5'. 


SUM  MARY 


Depth  to  packing 
in  feet 


1 46 
1 96 

247 


0 

20 

40 

60 

80 

40 

60 

80 

1 00 

1  20 

•  2S 

•37 

•5° 

.64 

■  79 

.  20 

•27 

•  3S 

.42 

•52 

.09 

.  1 2 

.16 

•  19 

•23 

100  =  Gage  pressure 

140  =  Calculated  pressure1 


1 .03  =  gallons  lost 
.67 

.28 


'Calculated  pressure  equals  average  pressure  plus  weight  of  column  of  water  from  surface 
to  ground  water  level.  Gage  pressure  is  given  in  pounds  per  square  inch.  Loss  is  in  gallo  n  s 
per  minute. 


1 62  NEW  YORK  STATE  MUSEUM 


4  HOLES  BORED  UNDER  AGREEMENT  NO.  45 


No. 
A4S 

Surface 

Rock 

Rock 

Per 

elevation 

elevation 

in 

penetrat'on 

cent  core 

Kind  of  rock 

feet 

feet 

in  feet 

saved 

ia 

426 

2 

278 

4 

19 

7 

O 

Slate  with  quartz 

2' 

39° 

5 

266 

0 

76 

0 

Slate 

3 

442 

8 

286 

8 

2S 

5 

0 

4 

43  2 

9 

268 

9 

31 

0 

s 

433 

307 

s 

3° 

5 

0 

«• 

6 

180 

1 

161 

8 

81 

7 

53 

Slate  with  quartz 

7 

179 

4 

163 

4 

26 

0 

0 

Slate 

8 

2  14 

2 

142 

2 

46 

5 

0 

Decayed  granite  gneiss 

9 

179 

4 

151 

9 

27 

5 

0 

Slate 

10 

260 

1 

237 

1 

34 

0 

0 

1 1 

2  I  4 

6 

155 

6 

95 

5 

0 

Decayed  granite  gneiss 

12 

237 

6 

236 

6 

35 

5 

0 

Slate 

13 

182 

7 

168 

6 

287 

4 

48 

14 

269 

s 

257 

3 

28 

0 

15 

209 

0 

!3° 

1 

36 

1 

0 

Decayed  granite  gneiss 

16 

213 

2 

1 1 

7 

308 

3 

28 

Decayed  granite  gneiss 
and  seamy  gneiss 

17 

387 

6 

387 

6 

163 

0 

69 

Gneiss  and  dyke  rock 

CHAPTER  IX 


ROCK  CONDITION  AT  FOUNDRY  BROOK 1 

Foundry  brook  is  a  small  stream  entering  the  Hudson  at  Cold 
Spring  in  the  Highlands.  It  drains  a  rather  abnormally  large  valley 
bordering  Bull  mountain,  and  Breakneck  ridge  on  the  east,  and  its 
axis  is  in  the  strike  of  the  principal  structure  of  the  gneisses  which 
form  the  chief  rock  formation  of  the  floor.  This  valley  is  in  exact 
line  with  the  course  of  the  Hudson  from  West  Point  immediately 
southward,  and  its  rock  formations  are  similar  in  character  and  con- 
dition. 

There  is  greater  variety  of  rock  composition  in  this  belt,  i.  e.  the 
Foundry  Brook-Hudson  river  belt,  than  in  any  other  in  the  High- 
lands of  similar  area.  The  eastern  half  of  the  belt  is  a  typical 
development  of  banded  gneisses  and  schists  and  quartzites  belonging 
to  the  sedimentary  representatives  of  the  Highlands  gneiss.  Small 
layers  of  interbedded  limestones  are  frequent  together  with  serpen- 
tine, and  mica  and  graphite  and  quartz  schists.  In  addition  along 
the  east  bank  of  the  Hudson,  they  are  profoundly  modified  by 
crushing  and  shearing  in  zones  that  trend  with  the  formation,  i.  e. 
in  a  direction  leading  toward  and  through  Foundry  brook  valley. 

The  west  side  is  much  less  variable  and  is  bounded  at  the  margin 
by  one  of  the  most  massive  types  of  the  region  —  the  Bull  mountain 
and  Breakneck  mountain  gneissoid  granites,  which  are  essentially 
the  same  as  that  of  Storm  King  mountain. 

But  additional  structures  enter  Foundry  brook  valley  from  the 
western  side  at  an  acute  angle  with  its  axis  and  formational  trend. 
These  additional  structures  are  two  well  marked  faults,  which  cross 
the  Hudson  —  one  along  the  precipitous  southeast  face  of  Crows 
Nest  and  the  other  along  the  southeast  face  of  Storm  King  moun- 
tain. These  are  the  most  pronounced  escarpments  of  the  whole 
region.  The  first  one  crosses  the  Hudson  between  Cold  Spring  and 
Bull  mountain  and  in  passing  northeastward  loses  much  of  its  in- 
fluence upon  topography  and  its  movement  is  probably  dissipated  in 
that  direction.  A  line  from  the  southeastern  face  of  Crows  Nest  to 
the  point  to  be  described  runs  n.  500  e. 

1  Explorations  at  Foundry  brook  were  clone  under  the  direction  of  Mr 
William  E.  Swift,  division  engineer,  now  in  charge  of  the  Hudson  River 
division  of  the  Northern  aqueduct. 

6  163 


164 


NEW   YORK  STATE  MUSEUM 


Explorations 

Foundry  brook  therefore  contains  structures  that  could  produce 
considerable  effect  upon  the  quality  and  condition  of  rock  floor. 
The  rock  floor  is  covered  with  heavy  bouldery  drift — thicker  on  the 
Bull  mountain  flank  than  in  the  valley  bottom  proper.  Where  the 
aqueduct  line  crosses  the  floor  is  at  an  elevation  of  200  feet  to  350 
feet  A.  T.   Hydraulic  grade  of  the  aqueduct  is  about  400  feet. 

The  lowest  bed  rock  found  along  the  line  is  182.3  feet  and  the 
channel  of  the  present  stream  coincides  with  the  preglacial  one  in 
that  portion  of  its  course.  There  are  two  secondary  channels  — 
probably  tributary  stream  channels  on  the  west  side.  One  of  these 
lies  under  70-80  feet  of  drift. 

Borings  were  made  for  the  purpose  of  determining  the  rock  floor 
profile  and  the  condition  of  bed  rock.  In  most  of  them  the  ordinary 
gneisses  and  granites  were  penetrated  in  normal  condition. 

But  in  a  few  a  very  unusual  condition  was  found.  Hole  no.  2  at 
el.  347  feet  near  the  west  or  Bull  mountain  margin  penetrated  49 
feet  of  drift  to  el.  298.  Then  the  drill  passed  into  gneiss  which  was 
at  the  top,  the  first  30  feet,  of  a  fair  quality.  This  is  shown  by  the 
core  recovered  —  the  first  12  feet -recovering  over  50^.  But  the 
percentage  of  recovery  rapidly  fell  off  —  amounting  to  only  36$  in 
the  first  50  feet.  Only  1  foot  of  core  was  recovered  in  the  next  30 
feet,  or  only  3^.  While  from  that  point  el.  220  feet  to  the  bottom 
of  the  hole  el.  51.8,  at  a  depth  of  295.7  feet  from  the  surface, 
nothing  but  fine  decomposed  matter  was  washed  up.  There  was  no 
core  at  all.  This  was  at  first  reported  as  sand  by  the  drillmen,  and, 
coming  at  a  time  when  exploration  of  deep  buried  gorges  was  the 
rule  at  other  points  of  the  aqueduct,  there  were  many  questions 
about  the  interpretation  of  this  new  hole,  the  first  assumption  of 
the  drillers  being  that  an  overhanging  ledge  of  a  very  deep  gorge 
had  been  penetrated  passing  through  it  into  river  sands  below.  A 
little  study  of  the  material  proved  that  this  view  is  untenable.  The 
sandy  wash  from  the  drill  is  true  disintegrated  gneiss  much  decayed 
and  dislodged  by  the  drill. 

But  the  meaning  of  it  and  the  extent  of  it  are  after  all  important 
additional  questions. 

Interpretation  and  further  explorations 

It  is  certain  that  the  soft  material  and  the  "  sand  "  reported  from 
this  boring  represent  rock  decay  induced  by  underground  water 
circulation.    Water  circulation  is  rather  free  as  is  shown  by  the 


NEW  YORK  STATE  MUSEUM 


fact  that  there  was  an  artesian  flow  from  this  hole  of  10  gallons  per 
minute  after  reaching  a  depth  of  80  feet,  which  increased  to  15 
gallons  per  minute  after  reaching  a  depth  of  253  feet.  This  under- 
ground supply  is  maintained  since  completion  and  the  pressure  is 
sufficient  to  raise  the  water  about  15  feet  above  the  surface. 

This  is  a  behavior  that  is  consistent  with  the  geologic  conditions. 
The  boring  has  no  doubt  penetrated  a  crush  zone  following  one  of 
the  faults  which  enters  this  side  of  the  valley.  The  crush  zone  dips 
steeply  and  the  boring  has  penetrated  the  hanging  wall  of  more 
solid  rock  in  the  first  50  feet  and,  after  reaching  the  broken  and 
decayed  portion  of  the  zone,  has  swung  off  parallel  to  the  dip  and 
avoiding  the  more  resistant  foot  wall  has  followed  down  on  the  soft 
inner  streak. 

a) 


Fi°  30  Sketch  illustrating  the  interpretation  of  geologic  structure  across  Foundry 
brook  valley  indicating  the  relation  of  certain  borings  to  them  and  their  supposed 
influence  in  deflecting  the  drills  ^^ttiimt^J  1 


This  crush  zone  extends  on  northeastward  across  higher  ground 
where  opportunity  for  taking  in  surface  water  is  offered.  This  is 
without  doubt  the  source  of  supply  for  the  circulation  which  fur- 
nishes the  artesian  flow  and  which  has  been  so  effective  in  pro- 
ducing decay  to  great  depth.  But  the  circulation  and  associated 
decay  are  probably  limited  to  comparatively  narrow  zones.  There 
is  no  good  reason  for  assuming  large  masses  of  rotten  gneiss  at 
great  depth.  The  worst  zones  are  narrow  but  may  be  comparatively 
deep,  i.  e.  they  may  extend  much  deeper  than  any  of  the  borings  yet 
made  in  this  valley.  The  depth  of  decay  is  related  to  the  outlet  for 
underground  circulation  which  in  this  case  is  the  gorge  of  the 
Hudson. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


167 


Several  other  boring's  encountered  similar  conditions,  especially 
those  on  the  west  flank  of  the  valley  within  range  of  the  belt  in 
which  the  fault  seems  to  be  located. 

Hole  no.  9  reached  the  rock  floor  at  a  depth  of  80  feet,  and  then 
penetrated  rock  to  a  depth  of  159.7  feet.  All  of  the  material  is 
badly  decayed.  Only  1  foot  of  core  was  recovered  from  the  whole 
boring  and  that  is  mostly  quartz  coming  from  a  veinlet  or  peg- 
matitic  streak  at  141  feet.  Water  under  slight  pressure  was  en- 
countered  in  this  hole  also.  But  because  of  the  somewhat  greater 
elevation  of  the  surface  at  this  than  at  hole  no.  2  there  is  not  a 
constant  outflow. 

Two  other  holes  immediately  to  the  west  show  much  better  rock 
condition  —  no.  1  showing  79$  core  recovery.  Also  two  on  the  east 
side  at  greater  distance  [see  accompanying  profile]  show  good  rock. 
But  one  other  no.  3  at  a  distance  of  over  a  thousand  feet  to  the  east 
encountered  another  zone  of  decayed  rock,  the  record  being  very 
similar  to  no.  2  in  that  poorer  conditions  are  shown  at  depth  than 
near  the  surface.  Rock  was  found  at  a  depth  of  20.2  feet.  From 
20.2  to  116  feet  the  gneiss  was  quite  hard,  55.3  feet  of  core  being 
recovered  or  S7-7/'-  But  from  116  feet  to  the  bottom  207.5  feet  the 
material  was  as  bad  as  in  hole  no.  2,  and  no  core  was  recovered. 

Several  other  tests  were  made  on  the  borings  with  a  view  to  de- 
termining the  character  and  extent  of  these  features  more  definitely. 
For  example,  if  the  interpretation  given  for  the  behavior  of  no.  2 
and  no.  3  is  correct  it  ought  to  be  possible  to  survey  the  holes  and 
determine  a  deflection  from  the  vertical  as  the  drill  deviated  from 
its  course  to  follow  the  softest  streak.  A  survey  conducted  for  this 
purpose  indicates  just  such  a  result.  The  accompanying  sketch 
shows  the  data  plotted.  The  drill  was  deflected  40  36'  at  a  depth 
of  50  feet,  70  36'  at  100  feet,  8°  2'  at  150  feet  and  90  40'  at 
198  feet. 

Pressure  tests  were  made  for  porosity  on  some  of  the  holes  in 
sound  rock.   Some  of  these  data  are  given  on  the  profile. 

Some  of  the  rock  of  this  valley,  if  very  extensive,  such  as  that 
in  borings  no.  2,  no.  3  and  no.  9,  would  be  very  poor  ground  for 
tunneling.  The  practical  question  involves  especially  the  width  of 
these  zones,  are  they  a  foot  wide  or  are  they  a  hundred  ?  In  an 
attempt  to  help  settle  that  question  an  inclined  hole  was  proposed 
that  was  to  run  at  an  angle  low  enough  to  crosscut  these  belts. 
Accordingly  hole  no.  T4  was  bored  inclined  400  26'  to  the  hori- 
zontal and  started  on  the  solid  gcanite  gneiss.    The  results  were  not 


i68 


NEW  YORK  STATE  MUSEUM 


Figure  31 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


169 


very  encouraging.  The  decay  is  shown  not  to  be  confined  to  mere 
seams.  The  doubt  raised  by  so  much  bad  ground  has  finally  led  to 
the  adoption  of  a  different  plan  for  crossing  Foundry  brook  valley 
and  no  further  data  are  likely  to  be  added  by  this  work.  As  it  now 
stands  the  borings  at  Foundry  brook  indicate  the  deepest  decay  of 
any  yet  made  in  granites  or  gneisses  except  those  of  Pagenstechers 
gorge  on  the  north  side  of  Storm  King  mountain.  Both  cases  are 
of  similar  origin  and  history,  but  Foundry  brook  is  apparently  the 
more  complex  in  occurrence.  There  are  several  parallel  zones 
along  which  there  is  extensive  decay  to  a  depth  of  more  than  300 
fc-et. 


CHAPTER  X 


GEOLOGY  OF  SPROUT  BROOK 

Three  creeks  unite  to  form  an  inlet  at  the  sharp  bend  in  the 
Hudson  immediately  above  Peekskill.  The  middle  one  of  these 
is  known  as  Sprout  brook.  It  occupies  a  deep  and  narrow  valley 
that  is  well  marked  for  10  miles  in  its  lower  course  and  is  trace- 
able as  a  physiographic  feature  of  less  prominence  to  the  north  mar- 
gin of  the  Highlands.  Its  persistence  indicates  some  important 
structural  control  in  erosion. 

Geology 

This  valley  lies  in  the  midst  of  the  most  typical  gneisses  and 
granites  of  the  Highlands  region.  And  in  addition  several  of  the 
"  iron  mines "  of  Putnam  county  lie  on  its  western  flank.  The 
rocks  are  complex  granitic  and  quartzose  gneisses  and  granites. 
Foliation  and  banding  and  bedding  wherever  this  appears  is  parallel 
to  the  axis  of  the  valley.  The  most  notable  geologic  feature  is  the 
occurrence  of  a  broad  belt  of  crystalline  limestone  throughout  the 
lower  4  miles.  It  is  undoubtedly  chiefly  this  limestone,  which  is  less 
resistant  to  weather  than  the  gneisses,  that  controls  the  form  and 
size  of  the  valley.  As  to  geologic  relations,  this  is  one  of  the  most 
interesting  formations  of  the  region.  It  is  coarsely  crystalline,  full 
of  silicious  impurities  at  many  places  and  carries  small  igneous  in- 
jections and  dykes,  and  so  far  as  the  bedding  can  be  followed, 
stands  almost  on  edge.  Although  an  actual  contact  is  not  seen,  at 
several  places  the  limestone  and  gneiss  approach  within  a  few  feet 
of  each  other  and  it  is  certain  that  no  other  formation  can  come 
between  them.  This  is  more  certainly  indicated  in  the  northerly 
extension  of  the  valley  where  the  limestone  gradually  disappears 
leaving  only  the  gneisses  and  granites.  That  there  may  be  a  fault 
contact  must  be  admitted,  but  of  this  there  is  no  good  evidence  in 
the  field. 

Such  relations  and  character  show  that  this  limestone  is  similar 
to  the  smaller  interbedded  occurrences  noted  frequently  with  the 
gneisses  in  the  Highlands  and  elsewhere.  If  it  is  of  that  type  then 
it  is  the  largest  representative  yet  found  in  that  series.  But  it  is 
also  in  these  characters  similar  to  the  Inwood  limestone  of  more 
southerly  areas.    The  overlying  Manhattan  schist  which  is  lacking 

171 


1/2 


NEW  YORK  STATE  MUSEUM 


may  have  been  removed  in  erosion.  One  of  these  types  it  resembles, 
but  it  can  not  be  the  Wappinger  (Cambro-Ordovicic)  as  was 
pointed  out  by  the  writer  in  a  former  report.1  The  Wappinger, 
wherever  known  to  be  such,  is  never  intruded  and  always  lies  above 
a  thick  quartzite  (Poughquag).  It  does  so  even  in  the  next  valley 
(Peekskill  creek)  less  than  a  mile  distant.  With  the  interpretation 
of  this  Sprout  Brook  limestone  therefore  is  involved  the  correlation 
and  interpretation  of  the  age  of  much  greater  areas.  That  the 
Sprout  Brook  limestone  is  not  Wappinger  is  clear  enough,  but  it 
could  be  either  interbedded  (Grenville)  or  Inwood.  If  it  is  Gren- 
ville  then  of  course  it  has  no  direct  bearing  on  the  Wappinger- 
Inwood  question  and  these  two  might  be  equivalents.  But  if  the 
Sprout  Brook  limestone  is  not  Grenville  (interbedded)  then  it  must 
be  Inwood  and  in  that  case  the  Inwood  and  Wappinger  are  not 
equivalent  —  which  means  that  there  are  two  series  above  the 
gneisses  instead  of  one  —  an  Inwood-Manhattan  series,  and  a 
Poughquag- Wappinger-Hudson  River  series.  At  the  present  time 
it  is  not  possible  to  give  with  certainty  a  final  interpretation  of  the 
Sprout  Brook  limestone. 

Explorations  2 

It  was  at  first  believed  that  a  pressure  tunnel  could  be  con- 
structed advantageously  at  the  point  of  crossing  this  valley  and 
borings  were  made  to  test  rock  conditions.  The  data  gathered  in 
exploration  are  indicated  on  the  accompanying  geologic  cross  sec- 
tion [fig.  32]. 

Borings  indicate  that  the  rock  floor  has  been  eroded  to  a  few 
feet  below  present  sea  level  and  that  the  gorge  has  a  drift  filling 
of  more  than  150  feet.  The  central  borings  penetrate  limestone 
and  indicate  a  total  width  of  this  type  of  more  than  400  and  less 
than  600  feet.  The  best  estimate  on  the  basis  of  these  explora- 
tions is  500  feet.  Whether  this  width  represents  one  thickness 
of  the  formation  as  would  probably  be  the  case  if  it  is  an  inter- 
bedded Grenville  layer,  or  part  of  a  double  thickness  due  to  infold- 
ing, as  would  probably  be  the  case  if  it  is  the  Inwood,  there  is 
no  evidence.  The  thickness  seems  to  be  even  greater  farther  south 
in  the  same  valley  (it  becomes  )A  mile  wide),  but  it  can  not  be 

1  Structural  and  Stratigraphic  Features  of  the  Basal  Gneisses  of  the 
Highlands.    N.  Y.  State  Mus.  Bui.  107  (1907).    p.  361-78. 

2  Explorations  at  Sprout  brook  are  in  charge  of  Mr.  A.  A.  Sproul,  division 
engineer  in  charge  of  the  Peekskill  division. 


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accurately  measured  and  there  is  no  way  of  guarding  against  repe- 
tition of  folds.  The  valley  floor  is  decidedly  terraced  at  an  ele- 
vation of  about  130  A.T.  One  side  is  limestone  and  the  other  is 
granitic  rock.  This  is  probably  a  local  mark  of  the  Tertiary  base 
leveling  work. 

Because  of  the  great  depth  of  this  narrow  gorge,  it  would  require 
a  500  foot  shaft  at  each  side  to  lead  from  hydraulic  grade  down  to 
a  safe  level  for  the  pressure  tunnel.  For  a  crossing  not  more  than 
2000  feet  long  this  is  excessive  and  the  cost  becomes  greater  than 
by  other  methods  of  construction.  Consequently  the  tunnel  plan 
has  been  abandoned  and  it  is  not  likely  that  further  data  bearing 
upon  these  questions  will  be  added. 


CHAPTER  XI 


STRUCTURE  OF  PEEKSKILL  CREEK  VALLEY 

Immediately  east  of  Sprout  brook,  described  in  the  previous  sec- 
tion, is  Peekskill  creek,  which  drains  the  largest  valley  emerging 
from  the  southern  margin  of  the  Highlands.  This  valley  as  a 
physiographic  feature  is  continuous  with  the  Hudson  river  gorge 
from  the  sharp  bend  at  Peekskill  to  Tompkins  Cove.  There  are 
important  structural  features  along  the  strike  of  this  valley  which 
extend  very  far  beyond  the  limits  of  Peekskill  creek  itself,  among 
which  are  strong  folding  and  block  faulting.  The  chief  fault  con- 
tinues to  the  southwest  with  still  greater  prominence  and  appears 
on  the  west  side  of  the  Hudson  in  the  escarpment  forming  the 
southeastern  margin  of  the  Highlands  continuously  for  many  miles 
into  New  Jersey. 

Near  the  Hudson,  Peekskill  creek  and  Sprout  brook  unite  and 
the  structures  and  formations  characteristic  of  each  valley  converge 
until  in  the  last  half  mile  of  their  united  course  rock  formations 
characteristic  of  Sprout  brook  lie  on  one  side  of  the  valley,  those 
characteristic  of  Peekskill  creek  on  the  other,  and  the  contact  which 
follows  the  divide  to  that  point  then  passes  beneath  the  waters  of 
Peekskill  inlet.  Because  of  the  block  faulting  which  has  carried 
down  overlying  formations  and  protected  them  from  the  total  de- 
struction characteristic  of  the  central  Highlands  region  this  valley 
is  of  unusual  interest. 

Explorations  1 

The  aqueduct  line  crosses  this  valley  about  3  miles  from  the 
Hudson,  and  in  determining  the  possibility  of  crossing  by  pressure 
tunnel  in  rock  a  considerable  number  of  explorations  were  made. 

Enough  has  been  done  to  outline  the  rock  floor  profile  very  defi- 
nitely and  to  determine  the  condition  of  the  formations. 

An  examination  of  the  drill  cores  and  records  of  explorations 
shows  the  following  facts  which  are  compiled  as  fully  as  possible 
on  the  accompanying  cross  section. 

Phyllite.  One  boring  (no.  1)  is  in  a  phyllite  whose  character 
and  relation  to  other  formations  leads  to  the  conclusion  that  it 

1  These  explorations  were  directed  by  Mr  A.  A.  Sproul,  division  engineer 
of  the  Peekskill  division  with  headquarters  at  Peekskill,  N.  Y. 

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NEW  YORK  STATE  MUSEUM 


belongs  to  the  Hudson  River  slate  series.  This  type  of  rock  forms 
the  whole  western  side  of  the  valley  for  several  miles.  Beds  stand 
on  edge  or  dip  steeply  southeastward  and  are  in  good  sound  physi- 
cal condition.  The  rock  is  everywhere  a  fine  grained  micaceous 
slate  or  phyllite  and  in  some  places  carries  pyrite  crystals.  It  is 
impossible  to  estimate  the  thickness  or  minor  structural  habits. 
But  it  is  clear  that  it  forms  the  upper  member  of  a  series  that 
has  a  synclinal  structure  and  therefore  the  belt  represented  by 
the  phyllite  marks  the  axis  of  the  syncline  although  the  chief  val- 
ley development  lies  wholly  to  one  side. 

Limestone.  Eleven  borings  (no.  2,  3D,  4  C,  11,  13  C.  18, 
22,  23,  25,  26  and  29;  are  in  limestone.  All  show  essentially  a 
very  fine  grained  close  textured  crystalline  gray  or  white  or  bluish 
rock  with  strong  bedding  standing  nearly  vertical  or  at  very  high 
angles.  This,  because  of  its  character  and  relation  to  other  forma- 
tions, is  regarded  as  the  Wappinger  limestone  —  a  formation  well 
known  north  of  the  Highlands,  where  it  is  at  least  1000  feet  thick. 
From  present  explorations  it  is  now  certain  that  a  belt  3250  feet 
wide  is  underlain  continuously  by  this  formation  standing  nearly 
on  edge.  Unless  repeated  of  course  this  would  represent  approxi- 
mately the  thickness  for  Peekskill  valley.  But  it  is  not  believed 
to  be  so  thick.  It  is  more  likely  that  there  is  a  threefold  occur- 
rence brought  about  by  close  isoclinal  folding  (a  closed  s-fold) 
as  seen  in  the  accompanying  cross  section.  This  view  is  supported 
by  at  least  one  occurrence  of  the  underlying  quartzite  member  near 
the  center  of  the  valley  at  a  point  a  couple  of  miles  farther  north 
On  the  line  of  exploration,  however,  none  of  the  borings  pene- 
trate any  other  formation  beneath.  Attention  is  called  to  additional 
structural  details  and  physical  conditions  in  a  later  paragraph. 

Quartzite.  One  boring  (no.  5)  is  in  a  quartzite.  It  i-~  very 
pure,  fine  grained,  closely  bound  and  typical  quartzite.  The  beds 
stand  almost  vertical  aud  the  whole  thickness  is  known  from  nearby 
outcrops  to  be  approximately  600  feet.  From  its  character  and  re- 
lations to  other  formations  it  is  regarded  as  the  Poughquag  —  a 
well  known  formation  of  the  north  side  of  the  Flighlands. 

Gneisses.  Five  borings  (no.  7  K,  9  B,  17,  27  and  28)  are  in 
gneisses.  These  are  to  a  considerable  extent  simple  granite  gneisses 
of  igneous  origin.  But  there  is  the  usual  variety  characteristic  of 
the  Highlands  gneisses  and  no  doubt  they  are  representatives  of 
the  great  basal  gneiss  series  that  is  elsewhere  referred  to  as  the 
equivalent  of  the  Fordham  of  New  York  city. 


GEOLOGY  OF  THE  NEW   YORK  CITY  AQUEDUCT 


177 


2  Stratigraphy 

This  is  therefore  the  rock  series  of  Peekskill  creek.  It  is 
the  only  locality  on  the  south  side  of  the  Highlands  where  all 
are  represented  in  complete  and  simple  form.  There  is  no  doubt 
that  it  is  the  Poughquag-Wappinger-IIudson  River  series,  in  spite 
of  the  complete  absence  of  organic  evidence.  A  similar  though 
not  so  complete  and  clear  occurrence  is  to  be  found  on  the  west 
side  of  the  Hudson  near  Stony  Point  and  Tompkins  Cove.  That 
is  a  part  of  the  same  structural  syncline.  It  is  probable  also  that 
the  phyllite  so  finely  developed  in  the  village  of  Peekskill  in  the 
next  small  valley  to  the  east  is  the  same.  But  outside  of  these 
occurrences  there  are  none  that  clearly  represent  this  same  series 
as  a  whole  and  in  the  same  condition. 

No  more  striking  example  of  this  fact  can  be  found  than  the 
adjacent  Sprout  brook  described  in  an  earlier  section.  There  coarse 
crystalline  and  injected  and  impure  limestone  occurs  alone  —  no 
phyllite  and  no  quartzite.  When  one  remembers  that  the  two 
occurrences  so  strongly  contrasted.  Sprout  brook  and  Peekskill 
creek,  converge  until  they  actually  unite,  and  still  preserve  their 
stratigraphic  dissimilarity,  without  any  adequate  structural  reason 
for  it,  the  only  conclusion  possible  is  that  the  two  occurrences  rep- 
resent two  entirely  different  series  of  formations. 

The  Peekskill  valley  series  is  Cambro-Ordovicic  in  age ;  what  is 
the  other?  It  is  older,  at  least  that  is  certain.  But  is  it  (the  Sprout 
Brook  limestone)  as  old  as  the  oldest  of  the  gneisses  themselves 
and  therefore  interbedded  with  them  representing  locally  the  Gren- 
ville  ;  or  is  it  intermediate  —  Postgrenville  and  Precambric  —  with 
which  possibly  other  occurrences  of  rocks  of  similar  habit  and 
equally  uncertain  relations  belong? 

It  is  on  the  general  similarity  of  this  occurrence  to  the  Inwood 
limestone  as  known  throughout  Westchester  comity  and  New  York 
city  that  a  tentative  intermediate  series  has  been  recognized.  This 
is  the  Inwood-Manhattan  series.  Whether  it  is  in  reality  a  separate 
older  series  is  not  regarded  as  proven.  But  for  engineering  and 
practical  purposes  the  distinction  is  a  good  one  and  eminently  ser- 
viceable. Further,  discussion  may  better  be  continued  in  a  different 
publication. 

3  Rock  surface 

The  bed  rock  surface  is  pretty  well  outlined  by  the  borings.  A 
profile  based  upon  them  seems  to  leave  no  unexplored  space  of  suf- 
ficient extent  to  admit  a  gorge  of  great  consequence  to  a  lower  level 


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NEW  YORK  STATE  MUSEUM 


than  is  already  shown  in  holes  no.  I  and  no.  n  [see  profile  and 
cross  section,  fig.  33].  The  elevation  indicated  by  no.  3  D  is  be- 
lieved to  be  misleading  because  of  the  use  of  a  drill  that  was 
capable  of  destroying  a  part  of  the  ledge  rock  that  would  usually 
core.  The  points  believed  to  be  weakened  by  structural  disturbance 
and  therefore  most  likely  to  be  attended  by  erosion  and  stream 
action  are  in  the  vicinity  of  hole  no.  11,  near  the  present  creek,  and 
hole  no.  25,  near  Peekskill  Hollow  road. 

4  Buried  channels 

From  the  accompanying  cross  section  it  will  be  seen  that  the 
drift  cover  is  more  than  100  feet  thick  over  large  portions  of  Peeks- 
•kill  valley.  The  rock  floor  in  the  middle  of  the  valley  averages 
approximately  25  feet  A.T.,  while  the  drift  surface  except  where 
cut  out  by  stream  erosion  is  at  about  125  feet.  In  the  rock  floor 
there  are  two  depressions,  the  large  one  wholly  within  the  lime- 
stone belt  and  the  smaller  following  the  limestone-phyllite  contact. 
There  is  not  much  difference  in  their  depth  so  far  as  explored,  but 
there  is  a  possibility  of  a  somewhat  deeper  notch  in  each  one.  The 
depth  to  which  some  of  the  limestone  beds  are  decayed  by  under- 
ground circulation  would  lead  to  the  belief  that  a  deeper  notch  may 
exist. 

The  drift  cover  is  chiefly  partially  assorted  sands  and  gravels  in 
the  central  portion  of  the  valley,  and  more  of  a  till  on  the  eastern 
valley  side.  It  is  noteworthy  that  the  present  Peekskill  creek  lies 
far  to  one  side  following  closely  the  phyllite  wall. 

5  Underground  water 

Present  elevation  above  sea  level  is  so  slight  that  there  is  appar- 
ently little  encouragement  of  deep  underground  circulation.  But 
ar  certain  points  the  rock  has  been  found  to  be  very  badly  decayed 
to  a  great  depth  —  to  at  least  200  feet  below  sea  level.  This  is 
believed  to  have  been  accomplished  chiefly  at  a  time  when  the  re- 
gion stood  at  a  higher  level.  Hole  no.  22  is  especially  notable  in 
this  connection.  A  comparison  of  the  figures  of  core  saving  is  one 
of  the  best  lines  of  evidence  on  this  question.  Wherever  data  are 
at  hand  the  percentages  of  saving  have  been  put  on  the  cross  sec- 
tion. Hole  no.  29,  for  example,  shows  a  saving  of  only  n<£  in  the 
lower  250  feet,  reaching  a  depth  of  297  feet  below  sea  level. 

The  present  water  table  profile  is  shown  on  the  cross  section.  A 
great  body  of  water  stands  in  the  assorted  sands  directly  upon  bed 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


179 


rock  forming  essentially  a  great  reservoir  of  supply  that  has  ready 
access  to  the  almost  vertical  limestone  beds.  This  will  give  a  maxi- 
mum water  supply  to  holes  that  penetrate  porous  or  broken  por- 
tions of  bed  rock.  The  attitude  of  all  strata  is  especially  favorable 
for  admitting  an  almost  inexhaustible  supply  from  a  considerable 
drift-covered  area  within  which  circulation  is  probably  very  rapid. 

•  6  Condition  of  rock 

All  strata  of  this  valley  stand  so  nearly  on  edge  that  drills  actually 
explore  a  very  limited  portion  of  the  whole  series  of  beds.  No  very 
great  advantage  is  gained  by  excessively  deep  boring  because  the 
drill  follows  necessarily  almost  the  same  bed  from  top  to  bottom. 
At  best  only  the  immediately  adjacent  beds  are  penetrated.  This 
means  that  much  of  the  total  thickness  of  beds  is  untouched  by 
present  explorations,  and  must  be  interpreted  on  the  basis  of  their 
general  likeness  to  those  more  fully  determined.  The  usual  suc- 
cession of  beds  is  known  to  be  quite  uniform  in  quality  and  loca- 
tions where  they  can  be  studied  and  there  is  no  reason  to  expect 
greater  variation  here. 

Deviations  from  such  normal  or  uniform  conditions  are  mostly 
due  (a)  to  local  development  of  mica  from  recrystallization  of  im- 
purities in  the  limestone,  and  (b)  to  crush  zones  developed  in  the 
process  of  folding  and  faulting  which  has  broken  the  rock  or  weak- 
ened it  enough  to  permit  a  more  ready  circulation  of  underground 
water.  Wherever  either  of  these  structural  conditions  prevail,  the 
rock  has  been  excessively  decayed,  or  disintegrated,  or  sufficiently 
weakened  in  its  binding  matter  or  its  sutures  to  crumble  in  the 
hand  or  break  down  to  a  sand  under  ordinary  boring  manipulation. 
This  condition  is  known  to  reach  to  -297  feet.  How  much  deeper 
is  not  known.  Probably  the  decay  dates  back  in  large  part  to  pre- 
glacial  continental  elevation  at  which  time  probably  there  was  more 
ready  deep  circulation  with  possible  outlet  in  the  Hudson  gorge. 
This  action  has  been  all  the  more  effective  by  reason  of  the  attitude 
of  the  beds.  They  stand  so  nearly  on  edge  that  they  present  all 
their  weaknesses  of  bedding  and  sedimentation  structures  to  the 
destructive  surface  agents.  They  admit  surface  water  readily  and 
favor  abundant  underground  circulation. 

Considerable  faulting  occurs.  The  contact  between  the  granite- 
gneiss  and  quartzite  is  a  fault  contact.  Wherever  seen  this  is  sound. 
But  a  crush  zone  in  limestone  lies  nearly  central  in  the  valley,  cut 
by  holes  no.  23  and  no.  25,  where  the  rock  shows  a  finely  brecciated 


NEW  YORK  STATE  MUSEUM 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


181 


condition  some  portions  of  the  drill  cores  being  literally  crushed  to 
hits. 

In  one  hole,  no.  ri,  near  the  phyilite-limestone  contact,  a  soft, 
sandy  condition  was  encountered  at  a  depth  of  133  feet,  permitting 
the  drill  rods  to  be  pushed  down  without  boring  at  all,  60  feet 
ahead  of  the  casing.  This,  however,  is  not  believed  to  indicate  any 
very  extensive  weakness.  It  is  probably  connected  with  the  bedding 
planes  or  joints  rather  than  with  general  decay  or  faulting.  Four 
or  five  inches  of  solution  and  disintegration  along  bedding  planes 
would  account  for  all  that  has  been  proven.  The  fact  that  the  rods 
could  be  shoved  down  60  feet  while  the  corresponding  outer  casing 
could  be  shoved  down  only  half  as  far  seems  to  support  this  view. 

Summary 

If  a  tunnel  were  made  across  this  valley  there  would  be  approxi- 
mately 1 100  feet  of  it  in  Hudson  River  slate  (phyllitc),  3250  feet 
in  Wappinger  limestone.  600  feet  in  Poughquag  quartzite,  and  the 
rest  in  the  gneisses. 

Some  weak  rock  is  certain  to  be  found,  especially  in  the  vicinity 
of  station  367+50  and  345+00  to  350-00.  At  both  places  increased 
water  inflow  would  be  encountered  with  almost  exhaustless  supply 
from  the  sands  that  lie  on  the  rock  floor  above. 

At  about  this  stage  in  the  exploration  the  Board  of  Water  Supply 
decided  to  abandon  the  rock  tunnel  plan.  The  conditions  found 
were  considered  by  them  too  questionable.  Steel  pipe  construction 
is  to  be  substituted.  As  a  result  it  is  not  likely  that  much  more 
detail  will  be  added  to  the  structure  of  this  very  complex  valley. 


CHAPTER  XII 


CROTON  LAKE  CROSSING 

It  is  proposed  to  finish  Ashokan  reservoir  and  the  Northern  aque- 
duct first.  This  so  called  Northern  aqueduct  reaches  from  the  Cats- 
kills  to  Croton  lake.  Croton  lake  is  the  present  supply  of  New 
York  city  and  is  already  connected  by  two  aqueducts  with  the  city 
distribution.  As  a  first  step,  therefore,  and  as  an  emergency  meas- 
ure the  Catskill  water  will  be  delivered  to  the  Croton  system  by 
finishing  the  Northern  aqueduct  first.  As  rapidly,  however,  as  the 
whole  project  can  be  carried  out  the  so  called  Southern  aqueduct 
will  be  constructed  to  continue  the  Catskill  water  independently  of 
the  Croton  supply  to  the  city. 

The  Southern  aqueduct  department  has  charge  of  the  line  from 
Hunters  brook  on  the  north  side  of  Croton  lake  to  Hill  View  reser- 
voir near  the  New  York  city  boundary.  During  exploratory  work 
it  has  been  under  the  direction  of  Major  Merritt  H.  Smith,  depart- 
ment engineer,  with  headquarters  at  White  Plains.  Construction 
now  going  on  is  in  charge  of  Mr  F.  E.  Winsor,  department  engineer. 

The  first  link  in  this  southerly  extension  is  to  be  a  tunnel  be- 
neath Croton  lake  through  which  the  Catskill  water  may  pass  in  the 
same  manner  as  it  crosses  other  valleys.  This  crossing  has  been 
located  a  short  distance  below  the  old  dam  on  the  Croton,  about  5 
miles  up  stream  from  the  Hudson. 

The  problems  involved  at  this  point  include  ( 1 )  a  determination 
of  the  kinds  and  quality  of  rock  to  be  penetrated,  (2)  their  water- 
carrying  capacity,  and  (3)  opinion  as  to  the  proper  depth  for  a 
successful  tunnel. 

Geological  features 

The  Croton  valley  is  one  of  the  very  few  in  southeastern  New 
York  that  actually  crosses  the  geological  formations  and  major 
structural  features  instead  of  following  parallel  to  them.  In  its 
lower  portion  it  passes  from  gneiss  to  limestone  and  to  schist  sev- 
eral times.  The  reason  for  this  somewhat  abnormal  course  is  prob- 
ably the  development  of  weak  zones  by  fault  movements  in  this 
transverse  direction. 

Only  one  of  the  well  known  formations  of  rock  is  exposed  in 
the  immediate  vicinity  of  the  tunnel  site.  This  is  the  Manhattan 
schist,  the  uppermost  formation  of  the  region  south  of  the  High- 
lands.   Along  the  Croton  it  varies  greatly,  the  chief  type  being  a 

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GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


l85 


garnet-bearing  quartz-mica  sohist  varying  from  rather  fine  grain 
and  semigrannlar  appearance  to  a  very  coarse  and  strongly  foliated 
structure.  This  part  of  the  formation  undoubtedly  represents  re- 
crystallized  or  metamorphosed  sediments.  But  associated  with  this 
fades  there  is  a  more  dense  black  hornblende  schist  that,  not  only 
here  but  at  many  other  places,  is  thought  to  represent  igneous  in- 
trusions that  have  been  metamorphosed  together  with  sediments  of 
various  types,  until  both  have  lost  almost  all  of  their  original  char- 
acters. The  hornblendic  schist  type  is  not  so  extensive  as  the  other, 
the  mica  schist,  but  it  is  more  compact  and  here  as  usual  is  in  the 
better  condition. 

Pegmatite  stringers  occur  abundantly,  especially  in  the  mica  schist 
varieties.  They  are  of  no  great  consequence,  however,  as  a  factor 
in  this  study.  They  originate!  in  the  aqueo-igneous  activity  in- 
volved in  the  reerystallization  of  the  rock  when  it  was  worked  over 
into  a  schist. 

Beneath  this  Manhattan  schist  formation  lies  the  Inzvood  lime- 
stone, a  bed  probably  at  least  70c  feet  thick.  But  at  this  point  how 
deep  it  lies  and  at  what  depth  it  would  be  penetrated  nobody  can 
tell.  None  of  the  drills  have  touched  it.  Beneath  the  limestone  in 
turn  lies  the  granitic  and  banded  gneisses  belonging  to  the  Fordham 
gneiss  series,  the  lowest  and  oldest  of  the  region. 

Along  the  Croton  river  nothing  but  Manhattan  schist  is  to  be  seen 
at  the  surface  for  more  than  a  mile  above  and  below  the  proposed 
crossing.  The  same  thing  is  true  for  an  equal  distance  on  opposite 
sides  from  the  river  at  this  locality. 

But  the  structure  is  folded  and  the  normal  northeast-southwest 
trend  of  the  folds  crosses  the  river,  every  arch  or  anticline  tending 
to  bring  the  limestone  and  gneiss  nearer  to  the  surface.  One  of 
these  folds  does  expose  the  limestone  and  gneiss  in  a  strip  extend- 
ing from  the  Hudson  river  northeastward  for  two  thirds  of  the 
distance  to  the  old  Croton  dam.  But  before  reaching  the  Croton 
valley  this  fold  pitches  down  toward  the  northeast  beneath  the  Man- 
hattan schist  and  passes  under  the  present  lake  (or  reservoir)  in 
diat  relation,  nut  reaching  the  Mirface  again  Eor  a  distance  of  about 
6  miles.  At  least  one  more  fold  is  known  to  behave  in  a  similar 
manner  as  it  reaches  the  Croton. 

These  facts  make  il  certain  that  there  is  limestone  beneath  the 
schist  in  the  vicinity  of  the  crossing,  and  that  it  comes  nearer  to 
the  surface  in  that  vicinity  than  at  some  other  places. 

South  of  the  Croton  there  are  several  small  cross  faults  run- 


NEW  YORK  STATE  MUSEUM 


ning  nearly  east  and  west.  It  is  believed  that  similar  movements 
have  affected  the  rock  in  the  Croton  valley  itself,  modifying  its  con- 
dition so  much  as  to  control  the  course  of  the  stream.  The  only 
immediate  bearing  upon  the  problem  of  the  Croton  crossing  is  the 
question  that  it  raises  about  the  quality  of  rock  and  the  necessity 
that  is  introduced  of  trying  to  determine  whether  or  not  there  is 
shattering  enough  to  be  very  objectionable. 

Explorations  and  data 

Six  drill  holes  have  been  made  on  this  proposed  Croton  lake 
crossing  —  one  on  either  side  just  at  the  margin  and  four  others 
within  the  intermediate  space  of  1400  feet.  These  inner  four  have 
been  made  from  rafts  floated  on  the  lake  and  have  penetrated  water, 
drift  cover,  and  rock  [see  accompanying  profile  and  cross  section, 
pi.  27]. 

Rock  floor.  The  depth  of  the  preglacial  Croton  valley  is 
pretty  accurately  determined  at  o  feet  or  sea  level.  There  is  no 
reason  to  expect  a  gorge  or  inner  channel  of  any  consequence. 

The  drills  have  penetrated  only  one  formation,  i.  e.  Manhattan 
schist.  These  test  holes  are  believed  to  be  near  enough  together  to 
eliminate  the  possibility  of  any  other  formation  appearing  at  tunnel 
grade. 

Rock  condition.  The  two  varieties  of  schist  (1)  the  coarse 
garnetiferous  quartz-mica  rock,  which  is  a  metamorphosed  former 
sediment,  and  (2)  the  darker,  close  grained  hornblendic  rock  that 
is  believed  to  represent  an  igneous  intrusion,  both  occur  in  the  cores 
brought  up  by  the  drill.  Either  under  normal  conditions  is  a 
good  rock.  But  there  are  considerable  differences  in  the  physical 
condition  of  the  rock.  Holes  no.  3  and  no.  4  at  the  two  extremes, 
on  the  lake  borders,  show  sound  rock  that  comes  up  in  large  cores 
with  very  high  percentage  recovery.  This  is  confidently  believed  to 
represent  the  average  condition  of  the  rock  in  this  vicinity  at  the 
sides  of  the  valley. 

The  central  holes,  however,  nos.  1,  2,  5  and  15,  all  show  more 
broken  ground.  Of  these  holes  no.  2  is  much  the  most  broken,  the 
core  recovery  being  only  14.8^.  The  pieces  are  small  and  many 
are  smoothed  (slickensided)  by  movement.  The  hole  penetrates  a 
typical  crush  zone  resulting  from  slight  faulting  movements,  and 
the  low  saving  is  due  to  the  fact  that  the  incipient  fractures  are  not 
well  bound  together  (rehealed)  by  later  mineral  change.  They  are 
probably  connected  with  the  latest  movements  of  this  kind. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


The  commonest  secondary  mineral  now  filling  these  crevices  is 
chlorite,  and,  although  it  may  completely  fill  the  crevices  it  has  little 
binding  strength.  Any  new  disturbance  or  strain  readily  causes 
separation  along  the  same  original  lines.  But  in  spite  of  the  fact 
that  the  core  is  broken  into  small  pieces  and  shows  so  low  percent- 
age of  recovery  it  is  quite  certain  that  the  rock  itself  is  not  badly 
decayed.  An  examination  of  one  of  the  most  doubtful  looking 
cores  from  the  lower  part  of  hole  no.  x  showed  under  the  micro- 
scope little  evidence  of  serious  decay.  This  is  believed  to  mean 
that  underground  water  circulation  is  not  as  abundant  as  the 
fractured  condition  of  the  rock  would  lead  one  to  expect.  Further- 
more, an  examination  of  the  cores  in  greater  detail  shows  beyond 
question  that  much  of  the  fracturing  is  entirely  fresh  and  must 
have  been  done  by  the  drill  itself.  It  is  certain  that  the  low  per- 
centage of  recovery  is  in  part  due  to  this  cause.  The  small  diam- 
eter of  the  intermediate  holes  is  contributory  to  the  same  results. 
Some  allowance  must  also  be  made  for  the  difficulty  of  working 
a  machine  from  a  raft  on  the  lake. 

Comparison  of  the  cores  shows  a  decidedly  higher  percentage 
of  core  recovery,  and  presumably  therefore  of  rock  solidity  in  all 
of  the  other  three  holes  —  no.  i,  no.  5  and  no.  15. 

Hole  no.    2  —  core   recovered  14.8^ 
"    no.    1  —  "  34-6$ 

"  no.  15—  36.3^ 
"    no.    5—  "  38.9^ 

It  therefore  appears  that  the  last  three  penetrate  rock  that  is 
more  than  twice  as  good  in  its  capacity  to  stand  drilling  disturbance. 

A  comparison  of  quality  at  different  depths  is  believed  to  be  still 
more  encouraging.  The  upper  portions  of  all  holes  have  poor 
recovery  and  comparatively  poor  looking  rock.  But  in  depth  there 
is  a  marked  improvement. 

In  view  of  the  fact  that  the  tunnel  will  undoubtedly  be  located 
somewhere  below  the  -75-foot  level,  it  is  really  only  this  lower  sec- 
tion that  is  of  vital  importance  to  the  project.  A  tabulation  and 
comparison  of  core  recovery  from  these  lower  portions  is  given 
below. 

1  From  total  depth  of  hole         2  From  depth  -75'  to  bottom 
Hole  no.    2  —   =  14.8$  core  recovery         25$  core  recovery 
"    no.    1—   =34.6^  "  450 

"    no.  15—   =36.3-1!  "  66> 

"    no.    5—    =38.9^  "  42^ 


NEW  YORK  STATE  MUSEUM 


Under  the  conditions  of  work,  this  is  a  fair  saving  and  indicates 
much  more  substantial  rock  below  the  -75'  level.  There  are  many 
pieces  10-12  inches  in  length  and  for  a  1  inch  core  this  may  be 
considered  very  good. 

It  is  clear,  however,  from  a  detailed  inspection  of  the  cores,  that 
there  is  considerable  variation  somewhat  independent  of  depth. 
There  are  occasional  stretches  of  poorer  ground  in  the  midst  of 
comparatively  sound  rock.  This  is  believed  to  indicate  that  the 
crushed  condition  is  confined  chiefly  to  certain  zones,  and  that  these 
zones  dip  across  the  formation  and  across  the  holes  at  an  angle. 
They  are  probably  distributed  promiscuously  throughout  the  central 
portion  of  the  valley,  but  are  certainly  more  abundant  and  more 
strongly  marked  in  the  vicinity  of  hole  no.  2  than  at  any  other  point 
tested.  The  rock  profile  shows  that  hole  no.  2  has  also  the  lowest 
bed  rock.  This  is  a  further  support  to  the  general  explanation  of 
the  valley  as  given  above. 

The  chief  elements  of  uncertainty  remaining  after  the  borings 
have  been  completed  are  : 

1  The  exact  extent  or  widths  of  the  chief  crush  zones 

2  Their  dip  and  strike 

3  The  possibility  of  others  not  yet  touched 

4  The  permeability  of  the  rock  for  underground  water 

5  The  supporting  strength  of  such  rock  in  a  tunnel  of  large 
dimensions 

In  spite  of  the  uncertainties  enumerated,  the  conditions  are 
entirely  understandable.  There  is  little  probability  of  finding  a 
worse  condition  than  that  shown  in  hole.  no.  2.  The  permeability  or 
porosity  of  these  zones  is  of  course  unknown.  The  chief  reason  for 
believing  that  underground  circulation  is  not  abnormally  heavy  is 
the  observation  that  the  joints  are  well  filled  with  chlorite  and  that 
other  decay  is  not  at  all  prominent  at  the  lower  levels.  Further- 
more, the  rock  is  a  crystalline  type  of  rather  successful  resistance 
to  ordinary  solution  agencies  and  therefore  may  be  depended  upon 
to  hold  its  own  in  its  present  condition  indefinitely.  But  because 
of  the  poor  binding  effect  of  the  chlorite  it  is  to  be  expected  that 
blocks  will  fall  from  the  roof  of  any  tunnel  where  it  passes  through 
a  crush  zone.  Timbering  will  be  required  for  protection  in  places, 
but  the  ground  will  not  cave  or  run.  These  zones  may  be  expected 
throughout  a  total  distance  of  about  700  feet  —  i.  e.  the  space 
between  no.  1  and  no.  15.  The  chief  belt  of  such  ground  probably 
lies  between  holes  no.  2  and  no.  5. 


GEOLOGY  OF  THE  NEW   YORK  CITY  AQUEDUCT 


Summary 

The  lowest  bed  rock  is  about  sea  level. 

This  pressure  tunnel  will  cut  only  Manhattan  schist. 

All  rock  is  good  ground  for  such  work,  except  in  certain  narrow 
zones  where  it  is  crushed. 

The  extent  of  such  broken  ground  is  not  closely  delimited,  but 
occurs  at  intervals  for  a  distance  of  700  feet. 

The  amount  of  underground  circulation  is  judged  to  be  moderate 
at  -100  feet. 

The  tunnel  should  be  located  deep  enough  to  take  advantage  of 
the  improved  rock  conditions  shown  at  about  -100  feet.  There 
seems  to  be  no  marked  improvement  below  -100  feet  as  deep  as 
the  drills  have  gone. 


•    CHAPTER  XIII 
GEOLOGY  OF  THE  KENSICO  DAM  SITE 

Kensico  reservoir  at  Valhalla,  2  miles  north  of  White  Plains,  is 
one  of  the  links  in  the  Bronx  river  aqueduct.  It  is  to  be  greatly 
enlarged  and  made  a  very  important  storage  reservoir  for  the  new 
Catskill  system.  In  line  with  this  plan  a  new  dam  is  to  be  built 
near  the  old  site  that  will  rise  100  feet  higher  than  the  present 
structure. 

Extensive  investigations1  have  been  made  to  determine  the  charac- 
ter of  rock  floor  for  this  massive  dam.  Sites  both  above  and  below 
the  present  one  have  been  studied  with  the  question  of  safety  and 
efficiency  and  permanence  as  well  as  that  of  economy  of  construc- 
tion in  view.  Involved  with  this  is  also  the  source  of  suitable  stone 
for  its  construction. 

Geological  surroundings 

Glacial  drift  covers  the  rock  floor  of  this  and  neighboring  valleys 
to  a  depth  of  10  to  20  feet.  No  rock  is  exposed  in  the  valley  bottom 
at  the  Kensico  site,  but  at  the  extremities  of  the  proposed  dam  the 
rock  floor  comes  to  the  surface  in  small  outcrops.  The  material 
constituting  the  drift  cover  is  essentially  a  loose,  somewhat  porous 
till  passing  into  modified  types,  especially  gravels  and  sands  imme- 
diately south  of  the  ground  tested. 

The  character  of  bed  rock  at  the  two  extremities  and  beyond  the 
limits  of  the  dam  is  easily  seen  from  the  outcrops  to  be  Fordham 
gneiss  on  the  east  and  Manhattan  schist  on  the  west.  Between, 
although  nothing  can  be  seen,  Inwood  limestone  is  found  by  the 
borings  as  was  to  be  expected.  No  other  formations  occur,  although 
the  Yonkers  gneiss,  an  intrusive  in  the  Fordham  at  a  little  greater 
distance  figures  prominently  in  studies  of  material. 

The  formations  are  in  normal  order  and  are  of  the  usual  petro- 
graphic  character.  All  dip  westward  at  angles  that  vary  from  45 
to  65  degrees  and  have  a  general  strike  a  little  east  of  north.  It  is 
evident  that  the  whole  series  represents  an  eroded  limb  of  a  simple 
fold. 

1  These  explorations  have  been  in  direct  charge  of  Mr  Wilson  Fitch 
Smith,  division  engineer,  whose  headquarters  for  the  Kensico  division  is  at 
Valhalla,  N.  Y-    Preparations  for  construction  have  already  been  begun. 

191 


192 


NEW   YORK  STATE  MUSEUM 


The  Inwood  limestone  occupies  about  800  feet  of  the  bottom  and 
eastern  margin  of  the  valley,  lapping  well  up  on  the  Fordham 
gneiss.   The  drill  cores  from  this  formation  are  unusually  sound. 

The  Manhattan  schist  shows  much  broken  material.  There  are 
many  crush  zones.  This  condition  increases  still  farther  west  along 
the  railway  near  Valhalla  station. 

The  Fordham  gneiss  appears  to  be  sound  where  it  can  be  seen 
at  the  surface. 

Results  of  exploration.  .Many  borings  have  been  made.  They 
prove  the  general  structure  and  succession  of  formations,  making 
the  boundaries  definite.  They  increase  the  evidences  of  a  rather 
wide  prevalence  of  weak  zones  —  some  of  them  in  the  gneisses. 
And  they  also  indicate  a  more  extensive  surface  decay  than  was 
formerly  believed  to  prevail. 

The  chief  problems  from  the  geologic  standpoint  are  connected 
with  the  following  features : 

1  Extent  of  surface  disintegration 

2  Extent  and  distribution  of  weak  zones 

3  Depth  of  decay  and  perviousness  of  rock 

Surface  disintegration.  Several  borings  on  ground  underlain 
by  Fordham  gneiss  penetrated  material  beneath  the  drift  and  above 
bed  rock  that  was  interpreted  as  residuary  matter  from  rock  decay. 
All  of  this  material  is  of  local  origin.  Later  exploration  in  the 
form  of  a  deep  trench  to  bed  rock  has  proven  that  there  is  an 
extensive  residuary  mantle  of  this  sort  at  the  eastern  side  of  the 
valley  below  the  present  dam.  In  places  as  much  as  30  feet  exists. 
Undoubtedly  this  material  is  a  remnant  of  preglacial  soil  mantle 
that  was  in  some  way  protected  from  removal  by  the  ice.  Few 
places  are  to  be  seen  in  all  southeastern  Xew  York  where  there  is 
so  much  left  in  place.  In  most  of  it  the  gneissic  structure  is  still 
preserved,  but  the  decay  is  so  complete  that  it  can  be  cut  and 
handled  like  an  impure  clay. 

Weak  zones.  It  has  been  proven  that  there  are  weak  zones 
in  the  gneisses  as  well  as  in  the  other  rock  formations.  In  some 
places  the  rock  is  so  poor  that  no  core  is  recovered  for  distances 
of  5  to  to  feet,  and  in  one  hole  a  seam  of  this  kind  20  feet  wide 
appears.  In  every  case,  however,  the  drill  passes  through  the  rot- 
ten material  into  the  opposite  wall  —  indicating  a  zone  of  consider- 
able dip  instead  of  vertical  position.  This  favors  the  theory  that 
the  weaknesses  follow  the  bedding  largely  and  are  perhaps  due  to 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


193 


difference  in  the  mineral  make-up  of 
the  beds  fully  as  much  as  to  dynamic 
disturbances.  The  walls  are  generally 
good.  The  fragments  of  core  are  not 
much  slickensided.  In  the  schist  this 
is  probably  not  as  generally  true. 
There  are  much  plainer  evidences  of 
crushing  movements  in  the  schist.  It 
is  a  locality  where  (me  of  the  folds, 
one  well  developed  farther  south,  is 
pinched  out  and  there  is  rather  gen- 
eral crushing  of  the  weaker  strata. 

Depth  of  decay  and  perviousness. 
As  deep  as  borings  have  gone  there 
is  occasional  decay  and  broken  ma- 
terial and  streaks  that  are  pervious. 

Final  location.  1  "he  condition  of 
bed  rock,  together  with  other  consid- 
erations led  finally  to  the  selection  of 
a  site  above  the  present  dam.  In 
general  the  same  features  character- 
ize this  site.  But  the  rock  condition 
is  somewhat  improved.  On  the  whole 
the  new  situation  is  a  safer  one. 


3    i  lO 


tih 


CHAPTER  XIV 


STONE  OF  THE  KENSICO  QUARRIES 

The  following  quarries  in  the  immediate  vicinity  of  Kensico  res- 
ervoir have  been  studied  in  the  field : 

(i)  "  Smith  quarry,"  which  is  less  than  a  thousand  feet  east  of 
the  southern  end  of  the  present  reservoir;  (2)  "  City  quarry,"  which 
is  on  the  immediate  eastern  margin  of  the  reservoir  on  the  east  side ; 
(3)  "  Garden  quarry,"  which  is  a  new  location  about  500  feet  from 
the  eastern  margin  about  midway;  (4)  "  Outlet  quarry,"  1500  feet 
east  of  the  northern  extremity  of  the  present  reservoir;  (5)  "  Ferris 
quarries  "  1000  feet  and  (6)  "  Dinnan  quarry  "  3000  feet  farther 
north. 

In  addition  to  the  field  observations  a  detailed  microscopic  study 
was  made  on  specimens  of  the  rock  taken  from  the  Garden,  Ferris 
and  Dinnan  quarries. 

The  question  at  issue  is  the  choice  of  a  rock  for  the  facing  and 
finish  of  the  new  Kensico  dam.  In  view  of  the  use  to  be  made  of 
the  rock,  extreme  strength  is  of  only  secondary  importance.  But  the 
questions  of  abundance,  distribution,  durability,  purity,  agreeable 
appearance  and  working  quality  are  vital. 

Types  of  rocks 

All  of  the  quarries  occur  in  the  broad  belt  of  Precambric  gneisses 
that  forms  the  eastern  margin  of  the  reservoir  extending  northward 
and  southward  for  many  miles.  The  formation  as  a  whole  is  very 
complex.  But  the  basis  of  it  is  a  black  and  white  banded  rock 
chiefly  a  metamorphosed  sediment,  known  as  the  Fordham  gneiss 
in  southeastern  New  York.  In  it  are  intrusions  of  igneous  rocks 
of  many  varieties  and  most  complicated  structure  —  dykes,  bosses, 
veinlets,  stringers  etc.,  sometimes  in  such  abundance  as  to  wholly 
obscure  the  original  type.  The  most  abundant  of  these  are,  (a)  a 
rather  light  colored  quite  acid  rock  that  is  essentially  a  granite  in 
composition,  but  has  a  sufficiently  foliate  structure  to  be  classed  as 
a  gneiss  and  is  the  same  as  the  "  Yonkers  gneiss  "  occurring  farther 
south,  and  (b)  a  dark  rock  containing  much  hornblende  and  biotite 
which  is  in  some  cases  essentially  a  diorite  in  composition,  but  has  a 
marked  tendency  to  schistose  structure.  The  former  (a)  may  be 
called  a  granite  gneiss  and  the  more  massive  representatives  of  the 
latter  (b)  may  be  classed  as  a  dioritic  gneiss.    In  both  cases  at 

7  I9h 


196 


NEW  YORK  STATE  MUSEUM 


times  the  blending  with  the  original  metamorphosed  Fordham  gneiss 
is  so  intimate  that  absolutely  sharp  limits  can  not  be  drawn.  And 
this  last  condition  may  well  be  designated  as  a  third  case  (c). 

The  quarries  visited  represent  all  three  of  these  cases.  Dinnan, 
Ferris  and  Outlet  quarries  represent  essentially  the  "  Yonkers 
gneiss"  type  (a)  of  granite  gneiss.  Garden  quarry  represents 
chiefly  (b)  the  dioritic  type  of  gneiss.  Gty  and  Smith  quarries 
represent  the  last  case  (c),  or  the  mixed  and  variable  type. 

Field  character 

City  quarry.  In  accord  with  the  above  differences  in  type  it 
is  found  that  large  quantities  of  uniform  material  for  such  purpose 
as  is  proposed  can  not  be  obtained  from  City  quarry.  The  rock 
there  is  badly  jointed  and  is  variable  to  a  marked  degree.  It  was 
not  thought  promising  enough  to  test  in  detail. 

Smith  quarry.  The  conditions  of  Smith  quarry  are  better  but 
there  are  similar  objections.  The  amount  of  uniform  material  is 
greater.  It  would  no  doubt  furnish  an  abundance  of  material  suit- 
able for  use  in  the  construction  of  the  dam  interior,  but  is  not  at 
this  point  as.  good  a  source  of  facing  stone  as  some  of  the  others 
to  be  considered. 

Outlet  quarry.  Although  this  rock  is  characteristic  Yonkers 
gneiss,  it  has  at  this  place  suffered  by  weathering  a  peculiar  dis- 
coloration to  such  extent  as  to  make  it  objectionable,  both  from  the 
standpoint  of  appearance  and  perhaps  of  durability. 

Garden  quarry.  There  is  an  abundance  of  stone  at  the  Garden 
quarry.  It  is  fairly  uniform.  It  is  no  doubt  good  enough  from 
every  standpoint  of  durability.  It  is  well  located.  It  can  be  quar- 
ried readily.  But  it  has  a  very  dark  color  and  is  undoubtedly  less 
attractive  than  a  light  stone  for  this  purpose.  There  are  no  objec- 
tionable structures,  except  where  the  strong  schistose  character  is 
developed,  and  these  could  be  avoided  so  that  with  a  little  selection 
a  fairly  uniform  stone  could  be  secured. 

Dinnan  quarry.  This  rock  is  typical  "  Yonkers  gneiss." 
There  is  sufficiently  large  quantity.  It  is  of  good  quality.  It  is 
situated  a  little  over  2  miles  from  the  proposed  dam,  but  is  of  easy 
access.  The  jointing  and  other  structures  do  not  seem  to  be  objec- 
tionable. It  will  work  somewhat  more  easily  than  a  true  granite 
because  of  the  gneissic  structure  and  it  has  a  good  medium  light 
color.  The  discolorations  do  not  seem  to  penetrate  deep  and  the 
rock  shows  only  slight  decay. 


Plate  28 


Photomicrograph  of  Yonkers  gneiss  from  "  Outlet  quarry "  taken  in 
plain  light  to  show  prominence  of  sutures  between  the  grains  indicating 
the  beginning  stage  of  disintegration.    Magnified  about  30  diameters 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  197 

Ferris  quarries.  The  "Old  Ferris  quarry  —  is  "  Yonkers 
gneiss  "  considerably  more  weathered  than  the  Dinnan.  It  is  con- 
sidered less  promising  than  the  "  New  Ferries  "  quarry  which  has 
been  explored  by  the  engineers  of  the  Kensico  division.  The  rock 
of  this  quarry  site  is  not  all  of  one  quality.  There  are  essentially 
three  varietal  facies  of  the  Yonkers  gneiss  type  and  relationship. 
One  (a)  is  essentially  a  granite.  It  has  a  coarse  grain  and  shows 
almost  no  foliate  structure.  It  has  a  decidedly  massive  appearance ; 
but  it  is  not  of  very  great  extent.  This  rock  is  evidently  very 
closely  related  to  the  true  Yonkers  gneiss  into  which  it  passes  on 
all  sides  through  an  intermediate  variety. 

This  intermediate  variety  (b)  has  medium  size  of  grain,  is  only 
slightly  foliated  and  passes  without  sharp  limitations  on  the  one 
side  into  the  granite  facies  and  on  the  other  to  true  normal  Yonkers 
gneiss.  It  is  not  so  strikingly  massive  as  the  granite,  but  is  more 
so  than  the  gneiss  proper.  This  rock  may  be  called  a  gneissoid 
granite  to  distinguish  it  from  the  other. 

The  true  Yonkers  (c)  gneiss  surrounds  these  two  special  varie- 
ties. It  is  of  finer  grain  than  either  of  the  others  and  is  more 
strongly  foliate  and  is  strictly  a  granite  gneiss.  Varieties  (a)  and 
(b)  occur  as  sort  of  a  lens  within  the  Yonkers  gneiss. 

The  extent  of  the  granite  as  now  uncovered  at  the  site  is  be- 
lieved to  represent  its  'limits.  The  prospect  of  enlarging  the  area 
will  not  meet  with  much  success.  It  is  essentially  a  local  develop- 
ment connected  with  the  differentiation  of  the  parent  magma  from 
which  all  three  varieties  were  derived.  It  seems  to  have  been  the 
last  of  the  three  to  solidify,  and  it  has  some  of  the  characteristics 
of  certain  pegmatite  lenses. 

Although  this  is  certainly  an  attractive  rock  and  one  against 
which  there  is  little  ground  for  objection,  it  is  reasonably  certain 
that  a  sufficient  quantity  of  this  variety  can  not  be  obtained  here 
for  the  whole  proposed  use.  And  the  prospects  are  not  good  for 
locating  another  quarry  of  the  same  quality. 

The  gneissoid  granite  (b)  is  of  greater  extent,  in  fact  it  will  be 
found  to  encroach  on  the  present  area  of  the  granite.  It  is  as  good 
rock  and  almost  as  attractive  as  the  granite. 

The  regular  type  of  Yonkers  gneiss  such  as  that  represented  in 
the  Dinnan  quarry  can  be  obtained  in  almost  unlimited  quantity, 
and,  with  the  splendid  showing  that  it  makes  in  further  examina- 
tion, it  has  come  to  be  considered  the  best  suited  to  the  purposes 
of  dam  construction  at  Kensico. 


NEW  YORK  STATE  MUSEUM 


Petrographic  character  of  the  rocks 

This  line  of  investigation  is  confined  to  four  sets  of  samples. 
No.  i  The  granite  of  the  New  Ferris  quarry 

2  The  gneissoid  granite  of  the  same  quarry 

3  The  Yonkers  gneiss  of  Dinnan  quarry 

4  The  dioritic  gneiss  of  Garden  quarry 

1  Granite.    The  rock  is  coarse  grained  and  well  interlocked. 
The  chief  constituents  are  orthoclase,  quartz  and  microcline. 
There  are  but  small  amounts  of  dark  minerals,  and  there  is  not 

much  decay. 

Both  surface  material  and  the  drill  core  were  examined.  The 
deeper  material  shows  a  little  calcite,  that  may  be  original,  occur- 
ring in  irregular  grains.  They  do  not  seem  to  indicate  decay. 
There  is  a  little  kaolin  alteration  of  the  feldspars,  but  not  to  a 
serious  degree.  There  are  no  injurious  impurities  in  the  rock  such 
as  might  cause  rapid  disintegration  or  discoloration. 

The  rock  is  undoubtedly  of  good  grade  as  to  strength,  composi- 
tion and  durability. 

2  Gneissoid  granite  (Ferris  quarry).  The  rock  is  of  medium 
grain,  containing  quartz,  the  feldspars  and  a  little  mica. 

There  is  very  little  alteration,  and  no  serious  decay  or  injurious 
constituents.  A  small  amount  of  seriate  and  calcite  present  are 
not  considered  of  consequence,  and  as  in  the  case  of  the  granite, 
the  calcite  is  believed  to  be  original. 

The  grains  are  intimately  interlocked  and  the  rock  is  certainly 
of  good  quality  and  very  similar  to  the  granite  proper. 

3  Yonkers  gneiss  (Dinnan  quarry).  This  rock  is  fine  grained, 
and  is  composed  of  quartz,  mica  and  the  feldspars  among  which 
microcline  is  very  abundant. 

The  condition  is  good, —  very  little  alteration,  close  structure,  but 
with  a  little  more  granular  appearance  than  any  of  the  other  types. 

It  is  a  good  rock  and  gives  good  durability  tests. 

On  badly  weathered  surfaces  the  Yonkers  gneiss  breaks  up  into 
a  granular  product  like  sand  long  before  it  decays  to  earthy  matter. 
This  seems  to  be  caused  by  expansion  and  contraction  of  the  dif- 
ferent constituents  under  changing  weather  conditions  inducing  a 
weakening  of  the  sutures.  Sometimes  there  is  very  little  decay 
even  along  these  sutures,  but  as  they  open  slightly  they  become  the 
channels  for  moisture  and  staining  solutions.  This  makes  the 
boundaries  of  the  grains  very  well  marked  in  weathered  specimens. 


Plate  29 


Photomicrograph  of  diorite  gneiss  from  "  Garden  quarry."  Magnifica- 
tion 30  diameters.  The  constituents  are  hornblende,  biotite,  feldspars 
and  quartz. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


199 


Such  incipient  disintegration  is  common  in  the  more  even  grained  or 
granular  varieties.  ' 

The  accompanying  photomicrograph  [pi.  28]  is  taken  in  plain 
light  and  shows  the  outlines  of  the  grains  due  to  this  cause. 

4  Dioritic  gneiss  (Garden  quarry).  Rock  is  of  medium  grain 
and  with  a  strong  tendency  to  schistose  or  foliate  structure.  The 
dark  grains  are  hornblende  and  biotite,  the  light  grains  are  feldspars 
and  quartz. 

The  rock  is  fresh,  durable  and  has  no  injurious  constituents.  It 
is  good  enough  for  the  use  in  all  respects,  but  has  a  dark  color  and 
is  more  strongly  foliated  than  any  of  the  others  considered. 

It  is  evident  from  these  observations  that  the  rocks  considered 
are  all  of  suitable  mineralogic  character  for  the  purposes  of  large 
dam  construction.  For  very  large  quantities  of  material,  however, 
it  is  probable  that  neither  the  coarse  granite  nor  the  gneissoid 
granite  could  be  depended  upon  for  uniform  supply.  The  true 
regular  Yonkers  gneiss,  however,  is  very  abundant,  and  can  be  relied 
upon  for  indefinite  amounts.  The  dioritic  gneiss  is  also  abundant. 
The  immediate  region  is  not  capable  of  furnishing  any  better  rock 
than  those  described  above. 

Additional  tests 

Some  instructive  tests  were  made  by  the  Board  of  Water  Supply 
under  the  direction  of  Mr  J.  L.  Davis  who  has  charge  of  the 
testing  laboratories.  A  few  of  these  applying  to  the  rocks  at 
Kensico  are  tabulated  below. 

The  tests  cover :  specific  gravity,  weight  per  cubic  foot,  porosity 
in  per  cent,  ratio  of  absorption,  per'  cent  water  absorbed,  ratio  of 
drying  24  and  48  hours,  retained  water  pounds  per  cubic  foot  24 
and  48  hours. 

In  the  accompanying  tabulation  the  terms  used  are  subject  to  the 
following  limitations  as  to  definition: 

1  Ratio  of  absorption,  sometimes  called  porosity,  "  is  the  ratio  of 
the  weight  of  water  absorbed  to  the  dry  weight  of  the  stone." 

2  Porosity  gives  "  the  actual  percentage  of  the  stone  which  is 
pore  space."  "  The  difference  between  the  dry  and  saturated 
weights  of  the  sample  is  multiplied  by  the  specific  gravity  of  the 
rock  and  the  product  added  to  the  dry  weight.  This  gives  the 
weight  the  specimen  would  have  provided  it  contained  no  pore 
spaces.    The  difference  between  the  dry  and  saturated  weights 


200 


NEW   VOKK  STATE  MUSEUM 


multiplied  by  the  specific  gravity  of  the  rock  is  then  divided  by  the 
above  computed  weight  of  the  poreless  specimen.  This  ratio  ex- 
pressed as  a  percentage  is  the  actual  porosity.  Expressed  as  a 
formula,  the  computation  is  as  follows: 

(Saturated  wt.  —  Dry  wt.)  S.  G. 

 — ■  — —  =  Porosity." 

(Saturated  wt.  —  Dry  wt.)  S.  G.  -j-  Dry  weight 

3  Ratio  of  drying.  An  attempt  has  been  made  to  determine  the 
comparative  and  actual  rates  at  which  the  saturated  rocks  give  up 
the  absorbed  water  under  ordinary  atmospheric  conditions.  "  The 
ratio  of  drying  was  computed  by  dividing  the  weight  of  water 
lost  during  exposure  by  total  weight  absorbed.  The  weight  of  re- 
tained water  was  computed."  The  comparison  is  most  useful  in 
rocks  of  like  petrographic  general  character. 

The  other  terms  need  no  explanation. 


TABULATION  OF  TESTS 


Name 

c 

e 

0 

CL 

:  absorp- 
er  cent 

per 
cent 

ravity 

per  cubic 

DOt 

water 
absorbed 

Ratio  of 
drying 

Retained  water 
pounds  per 
cubic  feet 

No.  of  s 

Ratio  01 
tion  p 

Porosity 

Specific 
g 

Weight 

f. 

Per  cent 

24 
hours 

48 
hours 

24 

hours 

48 
hours 

Granite,  Ferris 
quarry,  core  No.  461 

1 
2 

0.34 
0-31 

0.77 
0.84 

2  .66 
2  .65 

164.7  \ 
164.0  s 

0 . 26 

49-45 

52.8 

.  224 

.  210 

Gneissoid  granite, 
Ferris  quarry, 
core  No.  468 

1 

2 

0.32 
0.25 

0.81 
0.71 

2 . 63 
2  .65 

161 .0  ] 

162.8  J 

0.28 

67.48 

69.88 

.  146 

•  145 

Yonkers  gneiss, 
Dinnan  quarry 

1 

2 

0.30 
o.39 

0.87 
1  .01 

2 . 64 
2 . 64 

163 .3  1 
161 .0  J 

0.30 

88.16 

88.16 

•  057 

•  057 

Dioritic  gneiss, 
Garden  quarry, 
core  No.  459 

1 

2 

0 . 42 
0.24 

0.68 
0.68 

2.83 
2  .86 

175-4  ! 
174.8  1 

0.21 

62  .5 

62.5 

•  137 

•  137 

Gneissoid  granite, 
Ferris  quarry, 
surface 

1 

2 

0-37 
0.98 

0 . 96 
2  . 5° 

2  .63 
2.62 

162.5  1 
159-4  1 

1 .08 

86.7 

88.2 

.252 

.215 

Granite,  Ferris 
quarry,  surface 

1 
2 

0.44 
0.19 

1 . 1 2 
0 .50 

2  .63 
2.71 

162.3  1 
167-3  1 

0 . 40 

70 . 0 

74  -o 

.  207 

.  180 

Mr  Davis  concludes  from  a  careful  analysis  and  interpretation  of 
these  tests  that  the  Yonkers  gneiss  is  of  superior  durability. 


CHAPTER  XV 


THE  BRYN  MAWR  SIPHON 

Geologic  conditions  as  shown  by  exploration  for  a  proposed  pres- 
sure tunnel 

Bryn  Mawr  is  a  railway  station  2  miles  northeast  of  Yonkers. 
The  general  features  of  the  vicinity,  its  topography,  succession  ot 
formations  and  the  boundaries  are  shown  on  the  accompanying 
sketch  map  which  is  largely  copied  from  United  States  Geological 
Survey  Folio  No.  83.  The  Southern  aqueduct  follows  southward 
along  a  Manhattan  schist  ridge  until,  at  a  point  a*bout  a  mile  northeast 
of  Bryn  Mawr,  a  cross  depression  of  so  great  width  and  depth  is 
reached  that  some  special  means  of  crossing  has  to  be  devised. 
Near  Bryn  Mawr  station  a  gneiss  ridge  rises  and  continues  south- 
ward.  The  proposed  line  follows  this  ridge. 

Explorations  have  been  made  as  usual  by  drilling  to  determine 
if  possible  whether  or  not  a  bed  rock  pressure  tunnel  is  practicable. 

The  following  questions  may  be  made  to  cover  most  of  the 
practical  issues  of  the  study : 

1  What  formations  would  the  tunnel  cut? 

2  Which  of  these  would  show  most  questionable  ground? 

3  What  portion  of  the  line  is  regarded  as  most  critical  —  whose 
development  would  show  whether  or  not  a  tunnel  is  practicable  ? 

4  What  special  conditions  are  shown  by  drill  borings? 

5  What  interpretation  is  to  be  placed  on  the  peculiar  results  from 
hole  no.  4  where  there  has  been  unusually  great  difficulty  in  drilling? 

6  What  experiences  in  similar  ground  have  a  direct  bearing  on " 
this  case? 

Formations 

The  formations  that  would  be  encountered  in  the  Bryn  Mawr 
siphon  are : 

1  Manhattan  schist  (top),  the  usual  micaceous  type,  also  called 
1  Unison  schist  in  United  States  Geological  Survey  Folio  83. 

2  Inwood  limestone  (middle),  the  usual  coarsely  crystalline  dolo- 
mitic  and  micaceous  type,  also  called  "  Stockbridge  dolomite  "  in 
the  Folio,  same  as  "  Tuckahoc  marble,"  same  as  "  Sing  Sing 
marble,"  same  as  limestone  at  Kensico  dam  and  also  at  Croton  dam. 

201 


202 


NEW  YORK  STATE  MUSEUM 


3  Fordham  gneiss  (bottom),  the  usual  black  and  white  thinly 
banded  type,  a  much  folded  and  strongly  metamorphosed  rock,  the 
oldest  of  all. 

4  Yonkers  gneiss,  the  usual  type,  gneissoid  biotite  granite  very 
uniform  and  granular.  This  formation  is  an  igneous  intrusive  that 
cuts  up  through  the  Fordham  gneiss  and  is  therefore  younger. 
Whether  it  is  also  younger  than  the  limestone  and  schist  is  not 
clear. 

5  Quartz  veins  and  lenslike  segregations  of  quartz,  also  pegma- 
titic  streaks,  are  occasional  occurrences  in  all  of  the  formations. 
They  are  most  abundant  in  the  schist,  but  are  seen  also  in  the  Ford- 
ham gneiss.  A  similar  development  was  encountered  in  the  lime- 
stone in  hole  no.  40. 

6  Glacial  drift,  chiefly  modified  drift,  partially  stratified  sand 
and  gravel,  reaching  more  than  125  feet  in  depth,  covers  por- 
tions of  all  formations. 

This  last  formation  (no.  6)  is  the  only  one  that  may  be  wholly 
avoided  in  the  tunnel  proper.  The  chief  interest  lies  in  its  hindrance 
to  exploration  and  its  possible  usefulness  as  a  source  of  sand  and 
gravel  supply. 

Weakest  formation.  The  Inwood  limestone  is  the  most  ques- 
tionable ground.  This  is  believed  to  be  so  chiefly  because  of  the 
greater  solubility  of  the  rock,  its  granular  and  micaceous  character, 
and  the  probability  that  a  line  of  displacement  accompanied  by  some 
fracturing  crosses  the  siphon  line  in  this  formation.  If  a  very 
excessive  amount  of  shattering  occurs  in  this  zone  it  may  have 
induced  a  condition  of  disintegration  to  such  depth  as  to  endanger 
the  tunnel. 

There  are  no  surface  indications  of  a  serious  condition  at  depth 
for  any  of  the  other  formations. 

Critical  zone 

The  critical  zone  is  probably  not  far  from  the  contact  between 
gneiss  and  limestone.  There  are  two  reasons  for  this  opinion.  The 
first  is  related  to  the  nature  of  the  folding.  The  formations  are 
squeezed  into  a  close  syncline  pitching  northward.  In  cross  section 
the  strata  at  any  point  around  the  head  of  this  trough  dip  inward, 
and,  because  of  the  more  resistant  Fordham  gneiss  forming  the  floor 
of  the  trough,  the  drainage  and  seepage  and  consequent  tendency  to 
decay  might  be  expected  to  follow  along  its  upper  contact. 


Plate  30 


n\:-0/t)frf.  ftp**/?  secTwd 


Location  map  showing  by  the  dotted  belts  the  distribution  of  Inwood  lime- 
stone in  the  Hastings- Yonkers  district  and  the  position  of  the  Bryn 
Mawr  tunnel  section  as  well  as  shaft  13  on  the  New  Croton  aqueduct 
with  their  relations  to  the  limestone  belts.  Manhattan  schist  and  Ford- 
nam  gneiss  occupy  the  rest  of  the  area. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  203 

The  second  reason  is  related  to  the  probable  later  faulting  move- 
ments. It  is  evident  from  the  map  [Folio  83]  that  the  formations 
in  the  vicinity  of  Bryn  Mawr  are  bulged  up.  One  would  expect 
the  trough  which  contains  the  schist  and  limestone  of  Grassy  Sprain 
valley  to  continue  uninterruptedly  southwestward  and  join  with 
Tibbit  brook  valley.  But  a  cross  fold  has  bulged  the  formations 
up  so  much  that  for  a  distance  of  a  mile  erosion  has  removed  all 
of  the  formations  except  the  gneiss.  Bryn  Mawr  station  is  about 
central  on  this  bulge.  Evidence  of  such  a  movement  is  readily 
seen  on  the  gneiss  along  the  northerly  margin  where  it  slopes  down 
toward  the  limestone.  The  movement  had  developed  a  little  shear- 
ing and  has  tilted  the  minor  folds  downward  toward  the  north  at 
angles  varying  from  300  to  8o°  from  the  horizontal.  This  angle 
becomes  somewhat  more  accentuated  as  the  limestone  is  approached, 
and  it  is  believed  that  it  may  pass  a  short  distance  into  the  limestone 
border.  There  is,  however,  no  great  amount  of  crushing  evident  in 
the  gneiss  and  this  may  hold  also  in  the  limestone. 

The  fact  that  Sprain  brook  crosses  the  formations  along  this 
northerly  margin  and  flows  for  2  miles  in  a  southeasterly  direction 
may  indicate  a  still  later  movement,  probably  faulting.  There  is  no 
surface  evidence  of  it  except  the  abnormal  course  of  the  creek. 
But,  if  there  is  such  a  fault,  it  also  crosses  the  siphon  line  in  the 
same  zone,  i.  e.  in  the  vicinity  of  the  limestone-gneiss  contact,  not 
far  from  the  location  of  the  present  course  of  the  brook. 

Therefore  it  seems  reasonable  to  conclude  that  the  critical  zone 
is  near  the  contact,  probably  on  the  limestone  side,  and  in  the 
vicinity  of  the  present  course  of  Sprain  brook.  It  is  also  probably 
cut  deepest  here  by  erosion.  If  this  zone  is  in  good  enough  condition 
to  stand  tunneling  the  rest  of  the  line  ought  to  be. 

Conditions  indicated  by  borings 

All  rock  formations  stand  very  steep.  They  vary  from  8o°  to 
900.  This  means  that  very  few  beds  can  be  explored  by  one  hole, 
and  that  any  weakness  or  crevice  is  likely  to  make  a  showing  in 
excess  of  its  true  proportions. 

The  cores  show  considerable  crushing.  Some  of  the  fractures 
are  not  healed,  although  weathering  from  circulation  is  not  present 
on  all  of  them.  The  micaceous  layers  are  most  affected  by  circula- 
tion. Some  beds  of  this  variety  are  considerably  weakened  even  at 
depths  of  over  200  feet.  Occasional  seams  have  been  encountered 
that  give  no  core  at  all  for  several  (even  20  or  30)  feet.    But  the 


204 


NEW  YORK  STATE  MUSEUM 


greater  proportion  of  the  recovered  pieces  are  comparatively  solid 
even  where  the  total  percentage  of  saving  is  very  low.  It  is  evi- 
dent that  some  of  the  core,  a  considerable  percentage,  has  been 
ground  to  pieces  in  the  process  of  boring.  This  is  especially  notice- 
able at  hole  no.  40. 

Hole  no.  40.  Much  trouble  has  been  met  in  this  hole.  A 
careful  analysis  of  the  record  and  core  and  the  behavior  of  the  drill 
is  interpreted  as  follows: 

1  Partially  assorted  drift,  chiefly  sand  and  gravel  was  penetrated 
for  125  feet. 

2  Limestone  bed  rock  of  fairly  sound  quality  was  struck  at 
about  125  feet  (about  el.  -40). 

3  The  casing  that  was  put  down  to  shut  out  the  sand  failed  to 
reach  solid  rock,  and  this  permitted  a  continual  supply  of  pebbles 
and  sand  to  run  into  the  hole  and  obstruct  the  work  with  each  pull 
up.  The  presence  of  these  pebbles  was  also  instrumental  in  grinding 
the  core  to  pieces,  and  this  accounts  chiefly  for  the  low  saving. 

4  After  this  opening  was  plugged  up  with  cement,  the  drilling 
was  continued  successfully  until  a  somewhat  broken  quartz  vein 
was  encountered  and  this  has  been  followed  for  about  35  feet.  Its 
broken  condition  afforded  another  opportunity  for  fragments  to  fall 
into  the  hole,  and  on  top  of  the  drill,  bringing  the  work  for  a  second 
time  to  a  standstill.  It  is  certain  also  that  the  drift  pebbles  still  fall 
in.  As  the  formation  stands  vertical  here  it  is  not  surprising  that 
any  feature  should  show  an  apparent  extent  quite  out  of  proportion 
to  the  real  value.  The  quartz  vein  is  probably  of  no  great  breadth. 
Small  seams  containing  mud  may  also  be  followed  15  or  20  feet 
and  still  be  of  no  great  significance  in  the  formation  as  a  whole. 
The  rock  fragments  (core)  recovered  in  this  hole  are  fairly  sound. 

5  In  spite  of  the  many  delays  and  difficulties  of  this  hole,  it  is 
apparent  that  the  general  rock  formation  is  not  responsible  for  it 
all.  The  failure  to  reach  solid  rock  contact  with  the  casing  has  been 
the  cause  of  part  of  it.  Later  the  penetration  of  a  rather  rare 
quartz  vein,  a  thing  that  would  not  often  be  found  in  the  limestone, 
has  added  to  the  trouble.  Both  of  these  causes  are  so  rare  that 
they  may  almost  be  given  the  value  of  accidents. 

But  the  last  100  feet  or  more  of  the  hole,  from  depth  225  feet  to 
335  feet,  shows  an  unusually  questionable  condition.  Only  a  few 
rock  fragments  are  saved  and  they  include  limestone  and  quartz 
vein  matter.  The  rest  is  wholly  disintegration  sand  of  rather  com- 
plex composition  but  carrying  very  much  mica.    This  is  all  wash 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


2o6 


NEW  YORK  STATE  MUSEUM 


material  except  one  sample,  which  is  a  "  dry  sample  "  and  is  still 
more  strongly  micaceous. 

Borings  nos.  40,  45  and  46  are  all  within  the  zone  that  was  con- 
sidered, from  surface  indications,  to  he  likely  to  carry  the  deepest 
gorge  and  to  show  the  weakest  rock.  Because  of  the  heavy  drift 
cover  (more  than  a  hundred  feet)  it  is  manifestly  impossible  to 
locate  the  weakest  zone  more  closely  or  judge  of  its  exact  condi- 
tion except  by  borings. 

Hole  no.  42  at  station  634  +  28,  penetrates  82.4  feet  of  drift  and 
reaches  bed  rock  at  about  elevation  21  feet  A.  T.  The  rock  is  good, 
substantial,  coarsely  crystalline  limestone.  It  shows  as  sound  con- 
dition as  can  be  expected  in  this  formation  even  under  the  most 
favorable  situations. 

Hole  no.  46  at  station  644  +  77.4  is  just  south  of  the  brook.  It 
penetrates  72  feet  of  drift  and  reaches  bed  rock  at  elevation  14 
feet  A.T.  The  rock  is  Fordham  gneiss  of  typical  sort  and  in  per- 
fectly good  condition.  There  is  no  question  about  the  soundness  of 
the  rock  from  this  point  southward. 

Hole  no.  45  at  station  643  +  52.5,  125  feet  north  of  hole  no.  46 
penetrates  drift  for  about  150  feet  (possibly  a  few  feet  less,  145 
feet).  This  drift  cover  is  interpreted  as  mostly  sand  (modified 
drift)  to  115  feet  and  a  boulder  bed  from  115  to  143  feet.  After 
the  true  ledge  is  reached  it  is  sound  and  shows  no  unusual  or  ques- 
tionable conditions.    It  is  Fordham  gneiss. 

Interpretation 

1  Weak  zone.  There  is  little  doubt  that  this  last  100  feet  of 
hole  no.  40  is  in  the  decayed  weak  zone  that  was  expected  to  de- 
velop in  the  vicinity  of  the  contact  between  the  gneiss  and  the  lime- 
stone. It  would  be  expected  to  pitch  northward  along  the  floor  of 
gneiss  and  extend  beneath  the  southerly  extremity  of  limestone  at 
this  point  [see  fig.  36]. 

2  Contact.  Hole  no.  40  cuts  limestone,  hole  no.  45  cuts  only 
gneiss,  therefore  the  formational  contact  lies  somewhere  in  this 
177-foot  space. 

3  Position  of  old  channel.  Bed  rock  surface  is  lowest  at  hole 
no.  45.  But  since  the  rock  itself  is  sound  gneiss,  it  is  not  believed 
to  represent  the  lowest  possible  point.  This  is  still  more  certain 
because  of  the  fact  that  the  pitch  is  northward  so  that  this  becomes 
a  dip  slope  on  which  the  prcglacial  stream  could  glide  against  the 
edges  of  the  limestone  beds  [see  diagram],  and  because  the  condi- 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


207 


tion  of  the  rock  a  little  farther  north  (at  hole  no.  40)  shows  that 
these  limestone  beds  are  actually  much  weaker  than  the  gneiss. 
Therefore  the  deepest  portion  of  the  buried  channel  is  to  be  expected 
between  holes  no.  40  and  no.  45,  and  probably  nearest  to  hole  no.  40. 

4  Depth  of  old  channel.  How  deep  the  buried  channel  may 
be  can  not  be  accurately  estimated.  But  if  the  same  dip  slope  as 
is  shown  by  the  rock  surface  from  hole  no.  46  to  no.  45  prevails 
northward  toward  hole  no.  40,  a  depth  somewhat  below  -100  feet 
may  reasonably  be  expected.  In  the  absence  of  data  bearing  upon 
the  depth  of  other  portions  of  this  ancient  channel  or  of  the  lower 
Bronx  river  with  which  it  must  have  been  connected,  it  is  impossible 
to  estimate  more  closely. 

5  Interpretation  of  hole  no.  40.  There  is  so  little  rock  actually 
saved  from  the  more  than  200  feet  of  possible  core  on  this  hole 
that  its  real  character  is  very  obscure. 

There  are  three  possible  explanations  for  the  condition  found  in 
the  last  100  feet. 

a  The  drill  may  have  followed  a  large  mud  seam. 

b  The  material  may  be  only  residuary  rotten  limestone  still  wholly 
above  the  gneiss. 

c  The  actual  contact  may  have  been  penetrated  and  a  part  of 
this  rotten  material  may  be  decayed  gneiss  within  a  crush  zone. 

The  difficulty  in  drawing  absolute  conclusions  is  increased  by  the 
fact  that  matter  falling  in  from  above  has  been  a  continued  source 
of  trouble  and  is  more  or  less  mixed  with  the  rock  material  of 
lower  points.  Therefore,  the  fact  that  the  sand  taken  from  the 
lowest  points,  335  feet,  is  silicious  instead  of  calcareous,  may  not 
prove  satisfactorily  that  the  rock  at  that  point  is  wholly  silicious. 

It  is  worth  noting,  however,  that  the  harder  rock  in  the  upper 
portion  of  the  hole  was  in  places  much  crushed  and  that  mud  seams 
were  encountered  before  reaching  this  last  100  feet. 

It  is  also  worth  noting  that  the  same  dip  slope  of  rock  surface 
as  prevails  between  holes  no.  46  and  no.  45  if  continued  northward 
to  hole  no.  40,  would  cut  that  hole  a  considerable  distance  (75  feet) 
above  its  bottom. 

In  view  of  all  the  conditions,  therefore,  it  is  judged  that  there  is 
a  crush  zone  here,  that  hole  no.  40  penetrates  it,  that  it  is  badly  de- 
cayed, that  the  plane  of  the  crush  zone  dips  steeply  northward  and 
cuts  both  limestone  and  gneiss,  that  a  tunnel  at  about  -300  feet 
would  cut  this  zone  south  of  station  640  and  north  of  station  642, 
and  that  all  other  portions  of  the  line  are  in  comparatively  satisfac- 


NEW  YORK  STATE  MUSEUM 


tory  condition.  This  zone  for  a  hundred  feet  is  likely  to  be  wet, 
weak,  and  would  require  extra  precautions  and  additional  expense 
in  construction. 

6  Evidence  of  faulting.  Whichever  interpretation  of  hole  no. 
40  is  taken  is  in  support  of  some  displacement  in  the  nature  of 
faulting  between  holes  no.  40  and  no.  45.  If  the  gneiss  rock  floor 
is  not  reached  in  hole  no.  40,  then  the  greater  northward  slope  of 
it  from  hole  no.  45  to  no.  40  than  is  shown  from  no.  46  to  no.  45 
indicates  a  downward  movement.  If  on  the  other  hand,  the  iden- 
tity of  the  formation  in  the  lower  part  of  hole  no.  40  be  considered 
undetermined,  and  its  condition  attributed  to  decay  in  a  crush  zone, 
the  presence  of  the  crush  zone  itself  indicates  movement  of  a  fault 
nature. 

Conclusions  as  to  character  of  the  crossing 

In  considering  the  geological  conditions  as  a  factor  in  the  prob- 
lem of  practicability  of  a  tunnel,  it  is  necessary  to  note  the  follow- 
ing points : 

1  In  view  of  the  fact  that  the  deepest  point  in  the  ancient  chan- 
nel is  not  yet  found,  and  that  it  will  probably  go  below  -100  feet, 
it  would  be  necessary  to  figure  on  a  tunnel  grade  down  well  toward 
-300  feet. 

2  It  would  be  necessary  to  figure  on  a  wet  and  weak  zone  of  at 
least  100  feet  along  the  tunnel  and  a  more  expensive  construction 
at  that  point. 

3  The  ground  at  such  depth  south  of  station  642  is  unusually 
sound.  The  ground  north  of  station  636  may  be  counted  good. 
The  ground  between  636  and  640  may  be  considered  fair,  and  the 
ground  from  640  to  642  +,  troublesome,  containing  the  chief  ele- 
ments of  uncertainty. 

Fig.  36,  which  is  a  geologic  section  along  the  line  at  this  point, 
shows  the  distribution  of  these  features  drawn  to  scale. 


CHAPTER  XVI 


A  STUDY  OF  SHAFT  13  AND  VICINITY  ON  THE  NEW  CROTON 

AQUEDUCT 

[Sec  outline  location  map,  pi.  30] 

There  has  been  reference  made  occasionally  in  connection  with 
the  Bryn  Mawr  explorations,  as  well  as  others,  to  the  remarkable 
piece  of  bad  ground  encountered  in  1885  on  tne  New  Croton  aque- 
duct near  Woodlawn  in  the  Saw  Mill  valley.  This  experience  has 
been  the  source  of  much  misgiving.  Because  of  its  evident  im- 
portance and  close  relationship  to  conditions  that  may  exist  in  the 
same  formation  at  points  on  the  Catskill  line,  an  examination  of 
this  ground  was  made  for  the  purpose  of  comparison.  The  mean- 
ing of  that  case  and  its  bearing  on  the  Bryn  Mawr  questions  a~e 
given  below : 

Engineer's  records 

This  ground  and  its  remarkable  behavior  is  described  by  Mr  J.  P. 
Carson  in  the  Transactions  of  the  American  Institute  of  Mining 
Engineers,  September  1890,  pages  705-16  and  732-52. 

A  description  is  also  given  in  Wegman's  Water  Supply  of  the 
City  of  New  York,  1658  to  1895,  on  page  152. 

From  Mr  Wegman's  report  is  taken  the  following: 

The  south  heading  was  started  from  this  shaft  on  June  1,  1885. 
It  advanced  at  the  rate  of  about  80  feet  per  month  for  392  feet 
through  good  limestone  rock  (dolomite),  which  then  became  softer. 
On  December  9,  1885,  when  the  heading  had  reached  a  point  407 
feet  from  the  shaft  a  fissure  was  encountered  from  which  about 
100  cubic  yards  of  decomposed  limestone  clay,  sand  and  dirty  water 
poured  into  the  tunnel,  partly  filling  it  for  a  distance  of  125  feet. 
After  three  days  delay,  when,  only  clear  water  was  flowing  into  the 
tunnel,  the  fissure  was  plugged  with  straw.  The  heading  was  ad- 
vanced 20  feet  further  until  on  December  22,  1885,  an  outpour  three 
times  greater  than  the  first  occurred,  covering  everything  in  the 
heading  out  of  sight  *  *  *  borings  were  made  on  the  surface 
with  a  diamond  drill  to  determine  the  extent  of  the  soft  ground  in 
front  of  the  tunnel.    It  was  found  to  lie  in  a  pocket  in  the  rock, 

.209 


2IO 


NEW   YORK  STATE  MUSEUM 


which  had  a  length  of  no  feet  on  the  axis  of  the  tunnel  and  ex- 
tended for  a  short  distance  below  the  invert  of  the  conduit.  The 
soft  material,  consisting  of  sand,  gravel,  clay  and  decomposed  rock 
had  a  depth  of  about  160  feet  from  the  surface  to  the  top  of  the 
tunnel.  It  exerted  such  a  pressure  against  the  timber  bulkhead  that 
the  24-inch  oak  logs  used  as  "  rakers  "  (braces)  became  crushed  in 
24  hours  and  had  to  be  continually  renewed. 

The  chief  points  of  present  interest  are  that  the  tunnel,  at  a  depth 
of  about  160  feet  from  the  surface,  and  after  passing  through  sev- 
eral hundred  feet  (407  feet)  of  good  dolomite,  came  into  rotten  rock 
and  soft  ground  no  feet  across  on  the  line.  It  was  so  soft  that 
it  ran  into  the  tunnel  in  great  quantities  and  exerted  such  pressure 
as  to  make  progress  in  it  a  very  troublesome  and  costly  matter, 
taking  "  60  weeks  to  advance  the  tunnel  85  feet  "  and  costing  "  $539 
per  foot."  The  material  caved  in  so  freely  as  to  form  a  pit  on  the 
surface. 

Statement  of  geologic  conditions 

It  is  not  possible  to  interpret  the  conditions  at  this  locality  as 
fully  as  one  would  wish  because  of  the  vagueness  of  some  of  the 
statements,  but  the  following  facts  and  explanation  are  essentially 
correct : 

1  The  rock  is  the  Inwood  limestone,  the  same  kind  and  same 
general  conditions  as  all  of  the  limestone  belts  that  occur  in  the 
region  of  the  Southern  aqueduct. 

2  The  soft  ground  penetrated  at  the  point  in  question  —  407  feet 
south  of  shaft  13  —  called  in  the  Carson  report  and  others  "  a  fis- 
sure "  or  "  pocket,"  etc.,  is  in  reality  a  fault  crush  zone.  The  fault 
plane  probably  dips  steeply  southeast  and  strikes  n.  500  e.  cutting 
the  tunnel  line  at  an  angle  of  something  like  20°. 

3  The  point  is  well  up  on  the  side  of  the  valley  more  than  a 
hundred  feet  above  Saw  Mill  river,  and  the  strike  of  the  fault  zone 
in  its  southwesterly  extension  cuts  into  the  lower  portion  of  the 
valley,  so  that  underground  circulation  would  be  encouraged  along 
the  zone  in  this  direction. 

4  The  limestone  outcrops  very  near  by  on  the  west  side  of  the 
line  and  the  Manhattan  schist  occurs  near  by  on  the  east.  The  atti- 
tude of  the  beds  is  such  as  to  indicate  a  fault  of  the  thrust  type, 
The  accompanying  figure  illustrates  this  relationship  in  a  cross  sec- 
tion at  right  angles  to  the  axis  of  the  tunnel  [see  fig.  37] - 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  211 


Cvubh  ~Z-o  no.  S 

Fig.  37  Sketch  of  the  geologic  structure  at  shaft  13  on  the  New  Croton  aqueduct. 
Interpreted  from  field  observations 

5  It  would  appear  probable  that  this  zone  was  penetrated  at  the 
worst  possible  level,  i.  e.  near  enough  to  its  Wholly  decayed  upper 
part  to  furnish  no  resistance  at  all  to  the  overlying  sand  and  gravel, 
and  not  deep  enough  to  reach  the  more  substantial  (although  prob- 
ably crushed)  rock  that  may  reasonably  be  expected  to  prevail  at 
no  very  much  greater  depth. 

The  chief  point  is  that  the  weak  spot  has  a  reason  and  is  not  an 
accidental  thing  that  might  be  expected  just  anywhere.  But  it  must 
be  admitted,  in  spite  of  this  fact,  that  a  casual  examination  of  the 
locality  would  not  make  one  suspicious  of  its  existence,  and  it  is 
surprising  that  the  spot  could  have  caused  so  much  trouble. 

From  the  above  it  will  be  seen  that  in  several  respects  the  Bryn 
Mawr  case  is  somewhat  similar  to  this.  They  both  indicate  fault- 
ing ;  they  are  in  the  same  type  of  rock ;  they  both  show  or  indicate 
caving  tendencies. 

On  the  other  hand,  there  are  certain  elements  of  difference  some 
of  which  are  capable  of  very  materially  modifying  any  conclusion 
that  might  be  based  upon  the  simple  facts  of  likeness.  For  exam- 
ple—  it  should  be  expected  (i)  that  the  fault  movement  at  shaft  13 
would  be  the  greater  because  of  lying  in  the  more  prominent  lines 
of  such  displacement  of  the  region,  (2)  being  a  thrust  movement, 
the  crush  effect  is  probably  more  prominent  at  shaft  13  than  at 
Bryn  Mawr,  (3)  occurring  at  greater  elevation  above  probable  cir- 
culation outlet,  the  opportunity  at  shaft  13  for  extensive  and  rather 
deep  decay  is  the  greater,  (4)  being  cut  so  near  the  surface  (160 
feet),  its  condition  there  is  not  necessarily  a  reliable  guide  to  the 
seriousness  of  decay  at  a  greater  depth. 


212 


NEW  YORK  STATE  MUSEUM 


Comparison  of  Bryn  Mawr  and  shaft  13 

The  following  statements  embody  an  opinion  on  the  points  raised 
or  suggested  in  connection  with  a  reference  to  the  New  Croton 
difficulties  at  shaft  13.  The  items  are  therefore  treated  by  compari- 
son or  contrast  so  far  as  possible : 

1  Type  of  rock.  The  rock  explored  at  the  Bryn  Mawr  siphon 
is  the  same  formation  as  that  in  the  Saw  Mill  valley  cut  by  the 
New  Croton  aqueduct,  i.  e.  the  Inwood  limestone  —  sometimes 
called  "  Stockbridge  dolomite."  It  is  the  same  also  as  the  other 
large  limestone  belts  in  Westchester  county.  There  are  occasional 
small  strips  of  limestone  of  another  type,  but  its  behavior  could  not 
be  very  different. 

2  Soft  material.  "  Is  any  material  of  this  sort "  (like  that  in 
the  New  Croton  tunnel  near  shaft  13)  "  likely  to  be  encountered 
either  in  the  crushed  zone  at  boring  40  or  elsewhere  in  the  lime- 
stone belt?  " 

It  is  sure  to  be  encountered,  especially  near  hole  40,  if  that  zone 
is  cut  shallow.  The  behavior  of  the  lower  portion  of  this  hole  is 
very  similar  to  the  described  case  near  shaft  13.  The  only  prob- 
ability of  avoiding  it  lies  in  placing  the  tunnel  deep  enough  to  cut 
more  substantial  rock.  The  single  hole  upon  which  all  this  argu- 
ment is  based  can  scarcely  be  considered  a  thorough  enough  ex- 
ploration to  build  up  a  quantitative  statement  as  to  depth  or  width. 

There  is  no  evidence,  either  on  surface  or  in  the  exploration 
holes,  of  any  other  such  zone  on  this  line. 

3  Depth  and  extent.  Under  the  circumstances,  the  increased 
depth  makes  it  less  probable  that  so  much  ground  of  like  behavior 
would  be  found.  Again,  it  is  not  likely  that  precisely  the  same 
conditions  would  so  effectually  halt  operations  or  be  considered  so 
nearly  insurmountable  at  this  time.  One  of  the  many  serious 
objections  is  that  the  tunnel  would  have  little  strength  or  resist- 
ance to  a  bursting  pressure.  It  must  be  admitted  that  if  caving 
ground  were  penetrated  it  would  prove  very  difficult  to  handle  with 
the  gravel  cover  at  the  depths  now  considered,  i.  e.  300  feet  or 
more  below  the  surface. 

4  Water.  "  AVhat  are  the  probabilities  in  regard  to  the  quan- 
tity of  water  to  be  met  in  the  crushed  zone  near  boring  40?  Can 
any  limit  be  set  which  it  would  be  extremely  improbable  that  the 
inflow  would  exceed,  on  account  of  the  topography  of  the  country 
and  the  nature  of  the  overlying  materials?" 

There  is  likely  to  be  much  water.    Nearly  all  of  the  overlying 


GEOLOGY  OF  THE  NEW  YOKK  CITY  AQUEDUCT 


213 


drift  is  sand  and  gravel  that  is  probably  saturated  and  in  such  con- 
dition as  to  permit  easy  flow  to  any  lower  outlet.  It  may  readily 
carry  8-10  quarts  of  water  to  the  cubic  foot  or  about  2  gallons. 
The  area  covered  by  such  deposits  is  about  2500  feet  long  on  the 
southerly  base  along  the  creek  and  at  this  margin  is  approximately 
150  feet  deep.  The  northerly  margin  is  variable  and  reduces  in 
places  to  o  feet  in  thickness.  It  may,  however,  really  represent 
500,000,000  cubic  feet  of  this  gravelly  material  holding  1,000,- 
000,000  gallons  of  water  as  a  nearly  permanent  supply. 

This  overlying  material  is  necessarily  a  menace  of  no  mean  pro- 
portions. Every  crevice  or  crush  zone  remaining  unhealed  will 
have  water  and  plenty  of  it,  the  inflow  being  limited  only  by  the 
size  of  the  cracks  and  their  abundance  until  the  reservoir  should 
be  drained.   There  is  no  hardpan  bottom  to  act  as  a  dam. 

Outside  additions  to  this  permanent  supply  are  confined  to  that 
received  from  rain  and  the  stream.  The  rainfall  on  the  area  and 
immediately  available  as  addition  to  the  underground  supply  in  the 
lower  sands,  together  with  the  stream  flow,  which  would  probably 
sink  into  the  sands,  if  an  attempt  to  drain  the  underground  supply 
were  made,  may  be  expected  to  furnish  additional  water  at  a  pos- 
sible rate  of  2500  gallons  per  minute.  How  much  of  all  this  is 
available  at  tunnel  level  depends  wholly  upon  the  openness  of 
structure  in  the  rock.  There  is  nothing  else  to  materially  control 
the  permanent  and  additional  supply. 

There  is  evidence  in  hole  40  of  considerable  crushing.  That 
means  capacity  for  water  circulation,  but  how  much  no  one  can 
tell.  There  is  also  much  rotten  rock  in  the  same  hole.  This  means 
that  circulation  has  been  easy  and  effective,  but  how  much  now  no 
one  can  tell.  The  single  hole  (no.  40)  in  the  absence  of  any  other 
corroborative  data  is  not  sufficient  to  base  more  elaborate  or  precise 
quantitative  estimates  upon. 

5  Solubility.  What  is  "  the  nature  of  the  limestone  with 
reference  to  its  resistance  to  solution?" 

This  limestone  is,  as  all  limestones  are,  more  easily  attacked  by 
circulating  water  than  most  other  rock  types  [see  Rondout  Valley]. 
The  Inwood  limestone  such  as  occurs  at  Bryn  Mawr  is  crystalline, 
often  contains  much  mica  and  then  is  inclined  to  be  foliated  in 
structure,  and  it  prevailingly  stands  steeply  inclined.  Because  of 
these  features  in  which  it  differs  from  the  Rondout  Valley  lime- 
stones, it  is  likely  to  be  more  generally  affected  by  decay  along  the 
zones  permitting  circulation  than  any  of  the  Rondout  Valley  types. 


214 


NEW  YORK  STATE  MUSEUM 


The  Rondout  Valley  limestones  are  affected  along  joint  planes,  but 
the  effect  is  almost  wholly  confined  to  a  simple  enlargement  of  these 
crevices.  In  the  Inwood  an  additional  effect  is  the  weakening  of 
the  sutures  or  bond  between  the  individual  granules  resulting  in  a 
tendency  to  weaken  the  whole  mass  as  far  as  there  is  much  pene- 
tration of  seeping  water.  It  would  have  less  tendency  to  produce 
openings  or  caves,  but  greater  tendency  to  produce  a  rock  that 
would  crumble  in  the  hand  or  that  would  gradually  assume  the  con- 
dition of  a  lime  sand  or  a  micaceous  mud. 

As  to  the  effect  of  water  from  the  aqueduct  on  fresh  portions  of 
this  rock,  it  is  certain  that  the  rock  would  be  attacked  wherever 
exposed  to  direct  action.  Its  method  of  attack  is  by  solution,  and 
the  rate  of  attack  may  safely  be  reckoned  as  not  materially  different 
from  that  assumed  or  being  established  by  experiment  and  experi- 
ence on  the  Rondout  V alley  types. 

In  the  final  consideration  of  the  difficulties  at  Bryn  Mawr  the 
engineers  have  decided  to  abandon  the  tunnel  plan.  It  is  probable 
therefore  that  no  additional  explorations  of  direct  bearing  on  the 
problems  of  this  ground  will  be  made. 


CHAPTER  XVII 


GEOLOGICAL  CONDITIONS  AFFECTING  THE  LOCATION  OF 
DELIVERY  CONDUITS  IN  NEW  YORK  CITY 

Hill  View  reservoir  is  the  terminus  of  the  Southern  aqueduct. 
The  Catskill  water  is  to  be  delivered  at  this  point,  just  north  of  the 
New  York  city  line  on  the  Yonkers  side,  at  an  elevation  of  295 
feet.  From  this  reservoir  the  water  is  to  be  distributed  by  an  inde- 
pendent system  of  conduits  to  the  principal  centers  of  consumption 
in  lower  Manhattan  and  Brooklyn. 

It  is  believed  that  distribution  can  be  most  economically  made 
and  the  system  be  most  permanently  established  by  constructing 
the  main  trunk  distributaries  as  tunnels  in  solid  bed  rock  at  con- 
siderable depth  below  all  surface  disturbances. 

Preliminary  investigations  have  been  carried  on  by  Headquarters 
department,  Mr  Alfred  D.  Flinn,  department  engineer,  beginning 
in  1908.  As  the  active  work  of  exploration  was  entered  upon  Mr 
William  W.  Brush,  department  engineer,  was  assigned  to  this  special 
division  of  the  department's  work  and  most  of  the  preliminary  ex- 
ploration borings  were  planned  and  finished  under  his  immediate 
supervision.  With  the  resignation  of  Mr  Brush  to  take  the  post 
of  deputy  chief  engineer  in  the  Department  of  Water  Supply,  Gas 
and  Electricity,  Mr  Walter  E.  Spear,  department  engineer,  was 
secured  to  continue  the  difficult  work  of  finishing  explorations  and 
preparing  for  construction. 

Studies  of  conditions  affecting  such  a  system  and  explorations 
designed  to  test  the  ground  in  line  with  these  studies1  have  been 
made.  The  work  thus  far  done  in  an  exploratory  way  has  been 
confined  to  one  main  distributary. 

Section  A.    Preliminary  geological  study 

As  a  preliminary  step  toward  the  systematic  study  of  local  con- 
ditions affecting  possible  conduits,  trial  lines  were  laid  out  on  the 

xFew  engineering  enterprises,  probably,  have  been  planned  with  so  care- 
ful regard  for  all  known  geologic  conditions.  The  geologist  and  the  en- 
gineer worked  alternately  on  the  same  problems  until,  in  the  opinion  of  both, 
the  best  possible  line  was  selected.  It  is  the  writer's  belief  that  so  sys- 
tematic a  method  has  seldom  if  ever  been  carried  out  in  engineering  work 
of  this  kind.  On  this  account,  and  in  part  to  illustrate  some  of  the  pre- 
liminary stages  in  such  work,  many  of  the  original  facts  and  arguments 
and  suggestions  are  given  without  change  in  the  following  discussion. 

21S 


2l6 


NEW  YORK  STATE  MUSEUM 


city  map  from  Hill  View  reservoir  to  Brooklyn  by  three  different 
routes.  So  far  as  the  topography  and  city  development  and  other 
engineering  considerations  could  be  forseen  either  route  could  be 
u<ed.  Studies  of  all  kinds  were  expected  to  indicate  which  would  be 
the  most  favorable  and  whether  or  not  it  might  be  advisable  to  shift 
even  the  best  one  to  still  more  favorable  ground.  These  are  shown 
on  the  accompanying  map  which  also  covers  the  local  geology  of 
the  immediate  vicinity  of  the  lines  [see  pi.  32]. 

General  questions 

When  the  problem  of  the  practicability  of  a  rock  tunnel  for 
distribution  conduits  was  first  studied,  several  general  questions 
were  raised  which  indicate  the  lines  of  investigation  followed. 

1  What  is  the  character  of  the  rock  along  the  projected  conduit 
lines  shown  at  the  depths  required  for  such  tunnels? 

2  Will  the  rock  at  moderate  depths  be  such  as  to  permit  success- 
ful and  economical  construction  of  tunnels  to  be  used  under  the 
hydraulic  pressure  due  to  Hill  View  reservoir? 

3  Does  the  character  of  rock  in  the  vicinity  of  the  lines  vary 
sufficiently  to  materially  affect  the  cost  of  a  tunnel  if  the  lines  be 
shifted  approximately  1000  feet  either  way  from  those  shown  on 
the  original  map  as  trial  lines? 

4  Are  the  suggested  locations  of  conduit  lines  adapted  from  a 
geological  viewpoint  to  the  construction  of  pressure  tunnel  con- 
duits, and,  if  not,  what  changes  in  these  lines  would  be  advisable  ? 

5  Is  the  thickness  of  rock  covering  sufficient  at  all  points  to 
obviate  trouble  from  open  seams  and  disturbed  surface  rock? 

6  What  borings  and  other  field  investigations  should  be  under- 
taken to  determine  the  practicability  of  construction  of  pressure 
tunnels  along  the  lines  suggested? 

In  line  with  this  series  of  questions  a  thorough  geological  investi- 
gation was  begun,  the  chief  conclusions  of  which  are  given  below. 

Geological  formations 

There  are  six  local  formations  of  sufficient  permanence  and  in- 
dividuality of  character  and  of  sufficient  areal  importance  to  be 
treated  as  units  in  this  study.  These  are  described  in  some  detail 
in  part  1,  but  for  convenience  are  briefly  listed  as  follows: 

1  Glacial  and  postglacial  deposits  of  boulders,  clay  and  sand,  with 
silt  beneath  the  rivers. 


Plate  31 


A  relief  map  of  New  York  city  and  environs.    Reproduced  from  a  model 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


217 


2  Manhattan  schist,  the  most  abundant  formation,  chiefly  mica 
schist  with  very  subordinate  hornblende  schists,  and  usually  with 
abundant  pegmatite  lenses  and  veins. 

3  The  Inwood  limestone,  a  white,  dolomitic  marble  when  fresh, 
which  shades  into  impure,  micaceous  varieties. 

4  The  Fordham  gneiss,  varying  from  a  thinly  schistose  or 
quartzose  rock  to  a  strongly  banded  or  a  very  massive  and  much 
contorted  gneiss.    The  oldest  formation  of  the  district. 

5  The  Yonkers  gneiss,  an  original  intrusive  granite,  now 
squeezed  into  a  gneiss.    Younger  than  the  original  Fordham. 

6  The  Ravenswood  grano-diorite  or  as  it  might  be  called  in 
engineering  practice,  granite ;  an  original,  intrusive  rock  now  some- 
what gneissoid  from  pressure.   Younger  than  the  original  Fordham. 

The  Manhattan  schist,  the  Inwood  limestone  and  the  Fordham 
gueiss  are  cut  by  veins  or  dikes  of  coarsely  crystalline  granite, 
technically  called  pegmatite.  They  are  of  irregular  distribution  and 
do  not  affect  the  tunneling  operations  one  way  or  another. 

All  the  formations  older  than  the  glacial  drift  have  been  com- 
pressed into  a  series  of  northeast  and  southwest  folds,  and  all  have 
as  a  rule  a  steep  or  almost  vertical  dip.  The  axes  of  the  folds  are 
not  horizontal,  but  usually  pitch  downward  to  the  south  at  low 
angles.  Erosion  has  developed  a  series  of  ridges  trending  north- 
east and  southwest.  The  limestone  being  a  softer  and  more  easily 
eroded  rock,  almost  always  underlies  the  valleys  or  flats  and  the 
river  channels.   It  is  certain  also  that  there  is  some  faulting. 

Rock  at  depth 

The  distribution  of  geological  formations  along  the  proposed 
lines  has  been  shown  on  the  accompanying  map  [pi.  32].  In  gen- 
eral the  kind  of  rock  at  tunnel  depth  will  be  the  same  as  at  the 
surface  as  indicated  on  the  map  for  each  point.  Such  error  as  there 
is,  arises  from  two  causes :  (a)  Uncertainty  as  to  the  exact  location 
of  some  of  the  contact  lines  between  two  formations  (usually  due  to 
drift  cover),  and  (b)  dip  and  pitch  of  the  strata. 

In  the  first  case  (a)  where  the  drift  is  particularly  heavy,  it  is 
sometimes  impossible  to  fix  a  contact  line  accurately  from  surface 
features  alone. 

In  the  second  case  (b)  it  must  be  appreciated  that  nearly  all  of 
the  formations  dip  eastward  at  a  very  steep  angle,  so  that  a  form- 
ation would  usually  be  found  to  extend  a  little  further  east  at  depth 
than  at  the  surface.    And  also  all  formations  pitch  southward,  so 


2l8 


NEW  YORK  STATE  MUSEUM 


that  they  would  be  found  to  extend  considerably  farther  south  at 
depth  than  their  surface  outcrops.  This  angle  of  pitch  is  from 
io°  to  300. 

In  nearly  all  these  cases,  however,  the  obscurity  of  the  actual 
surface  boundaries  is  as  great  a  source  of  uncertainty  as  the  effect 
of  dip  and  pitch,  so  that  the  boundaries  as  mapped  may  be  con- 
sidered sufficiently  accurate  for  this  comparative  study  of  the  lines. 

It  is  worth  noting  that  the  rock  at  the  proposed  depths  of  tunnels 
would  be,  as  a  rule,  more  substantial  than  at  the  surface.  But  there 
are  several  places  on  all  of  the  lines  where  the  exact  condition  is 
unknown  at  the  surface  as  well  as  at  depth.  The  chief  points  of 
this  character  will  be  noted  in  a  later  paragraph. 

Comparison  of  lines1 

A  comparison  of  the  three  lines  submitted  as  the  basis  of  ex- 
amination—  (a)  the  westerly  one,  (b)  the  central  one,  (c)  the 
easterly  one  [see  accompanying  map,  pi.  32],  as  to  rock  formations 
likely  to  be  cut  by  them,  furnishes  the  following  figures : 

Line  A.    Going  southtvard  from  Hill  View  reservoir 

Feet 

6  200  Yonkers  gneiss  —  good  rock 
1  400  Fordham  gneiss 

1  400  Probably  largely  Inwood  limestone  with  one  weak  zone 

(at  Van  Cortlandt  lake) 
5  600  Fordham  gneiss  —  good  rock 

2  400  Near  contact  with  limestone,  probably  in  gneiss 
1  600  Crossing  Harlem  river  —  Inwood  limestone 

4  000  Inwood  limestone  —  probably  fairly  good  rock 
800  Inwood    limestone  - —  probably    containing    bad    zone  to 
Speedway 

16400  Manhattan  schist  (to  135th  st.) 
2- 000  Along  contact  between  schist  and  limestone 
4200  Inwood    limestone    with   one   weak  zone  (to  s.  end  of 
Morningside  Park) 

1  The  statements  of  quality  and  extent  of  certain  formations  and  zones 
are  capable  of  some  modification  as  exploratory  work  progresses.  Some  of 
these  are  noted  in  later  sections  of  this  report  under  special  headings,  such 
as  The  Lower  East  Side,  and  The  East  River-Brooklyn  section.  For  the 
present  purpose,  as  showing  the  development  of  the  geologic  basis  of  the 
project  it  seems  preferable  to  leave  the  accompanying  comparisons  in  their 
original  form  as  presented  to  the  board. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


219 


12  800  Manhattan  schist  probably  good  quality  (to  s.  end  of  Cen- 
tral Park) 

21000  From  Central  Park  to  East  river  —  no  outcrops  —  mostly 
Manhattan  schists  at  tunnel  depth.  Condition  largely 
conjectural1 — probably  mostly  good  rock  with  occasional 
weak  zones 

6000  Manhattan  island  to  City  Hall,  Brooklyn.  Containing  an 
unknown1  zone  in  the  East  river  and  unknown  quality 
of  rock  in  Brooklyn. 

Summary  of  Line  A 

Feet 

6  200  Yonkers  gneiss 

7000  Fordham  gneiss 

2400  Contact  (probably  in  gneiss) 
12000  In  wood  limestone 

2000  Contact  (  probably  in  limestone) 
29200  Manhattan  schist  (good) 
21  000  Estimated  Manhattan  schist  (fair) 

6000  Almost  unknown 


85  800  total 

Line  B.    Going  southward  from  Hill  View  reservoir 

Feet 

8  000  Yonkers  gneiss  —  good  quality 
13000  Fordham  gneiss  —  good  quality 
6  800  Inwood  limestone,  probably  mostly  in  fair  condition,  except 

at  two  points  (to  Cromwell  av.) 
6600  Inwood  limestone,  unknown  condition,  but  probably  largely 
poor  (to  Harlem  river) 
600  Inwood  limestone  —  unknown  condition  (Harlem  river) 
4600  Inwood    limestone  —  unknown   condition  —  probably  fair 
(to  Mt  Morris  Park) 
800  Manhattan  schist,  good 
800  Probably  Manhattan  schist  —  unknown 
2  800  Inwood  limestone  —  unknown  condition  ■ —  probably  at  least 

one  bad  zone  (to  106th  st.) 
12000  Manhattan  schist  along  Central  Park  —  good 

1  Explorations  since  conducted  by  the  Board  of  Water  Supply  have  proven 
the  quality  and  character  of  the  rock  floor  at  these  places.  For  the  revised 
statement  on  these  sections  see  the  special  discussions. 


220 


NEW  YORK  STATE  MUSEUM 


Feet 

8600  To  Broadway  —  Manhattan  schist    (little   known  except 

from  tunnels  already  made) 
14000  To  East  river,  prohobly  Manhattan  schist  (same  as  line  A) 
6000  Manhattan  island  to  City  Hall,  Brooklyn  —  uncertain  con- 
dition (same  as  on  line  A) 

SUMMARY  OF  LINE  B 

Feet 

8000  Yonkers  gneiss  —  good  quality 

13000  Fordham  gneiss  —  good  quality 

21400  Inwood  limestone  —  variable  quality 

12800  Manhattan  schist  —  good  quality 

23  400  Estimated  Manhattan  schist  —  fair 

6  000  Almost  unknown 


84  600  total 

Line  C.    Going  south  from  Hill  View  reservoir 

Feet 

6  000  Yonkers  gneiss  —  good  rock 

17400  To  Webster  av. —  Fordham  gneiss  — good  rock 

5  000  Along  contact  between  limestone  and  gneiss 

9800  To  138th  st. —  Inwood  limestone  with  probably  two  bad 
zones 

1  800  To   Bronx   kills  —  along  contact  between  limestone  and 
gneiss  —  uncertain  quality 
600  Across   Bronx  kills  —  mostly   in   limestone   containing  a 
fault  zone  —  probably  bad  ground 

6  400  Crossing  Randall's  and  Ward's  islands  and  Little  Hell  Gate. 

Nearly  all  is  Manhattan  schist  of  good  quality 
1  000  Crossing  Hell  Gate  —  Inwood  limestone 
1  200  Crossing  Hell  Gate  —  Fordham  gneiss  of  good  quality 
1  800  Astoria  point  —  probably  Fordham  gneiss  of  good  quality 
1  000  Crossing  another  limestone  belt 

1  000  To  Vernon  av. —  Fordham  gneiss  of  unknown  quality  con- 

taining one  fault  zone 

7  000  To  Nott  av. —  Ravenswood  grano-diorite  —  good  rock 

2  800  To  Borden  av. —  Probably  Ravenswood  grano-diorite. 
18400  To  Fort  Greene  Park  Brooklyn  —  almost  wholly  unknown 

but  contains  probably  5000  or  6000  feet  of  poor  ground 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


221 


SUMMARY  OF  LINE  C 

Feet 

6  000  Yonkers  gneiss  —  good  quality 
17400  Fordham  gneiss  —  good  quality 

6800  Along  contact  between  limestone  and  gneiss  (questionable) 
12400  Inwood  limestone  —  with  several  bad  zones 

6  400  Manhattan  schist  —  probably  good  quality 

3  000  Fordham  gneiss  —  probably  good  quality 

1  000  Fordham  gneiss  —  unknown  quality 

9  800  Ravenswood  grano-diorite  —  mostly  very  good  rock 
18  400  Almost  wholly  unknown 


81  200  total 


Tabulated  summary  —  Types  of  rock  formations 


LINE  A 

(west) 

line  b  (central) 

LINE  C  (EAST) 

Per 

Per 

Per 

Feet 

cent 

Feet 

cent 

Feet 

cent 

(7-2) 

8000 

(9-4) 

6  000 

(7-3) 

(8.1) 

13  OOO 

(IS- 3) 

21  400 

(26.3) 

.  ,    4  4OO 

(5-1) 

0 

(0) 

6  800 

(8-3) 

12  000 

(13-9) 

21  400 

(25-3) 

12  400 

(15.2) 

SO  200 

(58.5) 

36  200 

(42.6) 

64OO 

(7-8) 

Ravenswood  grano-diorite 

0 

(0) 

0 

(0) 

9  800 

(I2.0) 

6000 

(7-0) 

6  000 

(7-0) 

18  400 

(22.6) 

8e  8nn 

8,1  600 

8l  200 

Summary  of  quality 

LINE  A 

LINE 

B 

LIN  F 

c 

Per 

Per 

Per 

Feet 

cent 

Feet 

cent 

Feet 

cent 

Good  rock,  1st  grade..  . 

.  .  42  400 

(49-4) 

33800 

(40.0) 

39  800 

(49.0) 

Probably  fair,  2d  grade. 

...  30  800 

(35-9) 

34  800 

(41. I) 

13  600 

(l6.7) 

Probably  poor,  3d  grade. . 

. .  6  600 

(7-7) 

10000 

(II. 8) 

9  400 

(11. 6) 

.  .  6  000 

(7.o) 

6  000 

(7-1) 

18  400 

(22.7) 

85800 

100. 0 

84600 

IOO. 0 

8l  200 

100.0 

Argument  on  choice  of  line 
In  judging  the  quality  of  rock  and  its  suitability  for  this  con- 
duit the  factors  of  most  weight  are  the  same  as  those  repeatedly 
mentioned  in  connection  with  other  portions  of  the  Catskill  aque- 
duct line.  That  is,  in  brief,  that  the  harder  crystalline  rocks  of  the 
Fordham  gneiss1  and  Manhattan  schist  types  wherever  known;  to  be 


222 


NEW   YORK  STATE  MUSEUM 


free  from  fault  crushing  and  surficial  weathering  are  the  best 
variety ;  that  the  more  heavily  buried  areas  of  these  rocks,  together 
with  those  limestone  areas  that  are  known  to  be  the  most  substan- 
tial of  its  class,  should  be  regarded  as  fair  or  second  grade;  that 
the  more  obscure  areas  of  limestone  and  all  portions  crossing 
faults  or  rivers  or  crush  zones  in  any  rock  must  be  regarded  as- 
poor  or  third  grade.  This  rating  is  based  wholly  on  rock  char- 
acter and  without  any  consideration  of  cost  of  construction. 

From  the  above  it  is  clear  that  line  A  has  more  "  first  grade  " 
rock  than  either  B  or  C  and  less  "  third  grade  "  ground. 

Line  C  has  three  times  as  much  "  unknown  "  ground  as  either 
B  or  C  and  less  "  first  "  and  "  second  grade  "  rock. 

In  other  words,  the  three  lines  are  estimated : 


First  grade  rock  

Second  grade  rock  

First  and  second  grades  together 

Third  grade  rock  

Unknown  ground  


LINE  A 

LINE  B 

LINE  C 

Per  cent 

Per  cent 

Per  cent 

49.4 

40.0 

49.O 

35-9 

41.  I 

16.7 

85.3 

8l.O 

65-7 

7-7 

11. 8 

11. 6 

7.0 

7-i 

22.7 

In  addition  to  these  differences  of  quality,  it  appears  from  a 
study  of  the  areal  geology  along  the  respective  lines  that  a  tunnel 
would  pass  across  limestone  contacts  from  one  formation  to  an- 
other six  times  on  line  A,  four  times  on  line  B,  and  seven  times 
on  line  C.  These  may  all  be  considered  points  of  probable 
weakness. 

All  of  the  lines  cross  belts  of  well  known  weakness  believed  to 
represent  fault  zones.  Line  A  crosses  three  such  zones,  line  B 
crosses  two,  and  line  C  crosses  at  least  three. 

Furthermore,  all  of  the  lines  cut  limestone  for  greater  distances 
than  seems  desirable  or  necessary.  The  weakest  ground  and  the  most 
uncertain  quality  of  ground  that  can  be  mapped  falls  within  the 
limestone  areas.  In  this  respect  line  A  with  13.9$  of  limestone 
ground  is  preferable  to  line  B,  with  25.3$  or  line  C,  with  15.2$. 

From  the  above  it  is  apparent  that  line  C  is  least  defensible. 
Line  A  has  some  advantage  over  both  of  the  others,  especially  in 
quantity  of  first  grade  rock  quantity  of  first  and  second  grade 
together,  low  amount  of  the  known  poorest  grade  and  small  extent 
of  the  so  called  "  unknown  "  ground. 

The  chief  advantage  of  line  A  over  line  B  lies  in  its  much 
smaller  limestone  area  (12,000  feet  7'.s\  21,400  feet  or  13.9^  vs. 


GEOLOGY  OV  THE  NEW  YORK  CITY  AQUEDUCT 


223 


25.3$),  and  the  chief  advantage  of  line  A  over  line  C  lies  in  its 
much  smaller  amount  of  "  unknown  "  ground  (6000  feet  vs.  18,400 
feet  or  7.0^  vs.  22.6$).  On  these  grounds  line  A  is  the  least  ob- 
jectionable of  the  three  lines  proposed. 

But  it  is  also  clear  from  an  examination  of  the  field,  as  is  shown 
on  the  accompanying  map  [pi.  32],  that  it  is  possible  to  avoid 
some  of  these  objectionable  features  or  certain  parts  of  them  and 
materially  improve  the  figures  by  shifting  the  line  to  a  sort  of  com- 
promise position  between  line  A  and  line  B.  This  compromise 
line,  or  the  trial  lines  from  which  the  final  tunnel  line  may  result, 
should  follow  as  closely  as  possible  the  gneiss  and  schist  ridges 
and  should  avoid  the  limestone  areas  and  known  weak  zones  wher- 
ever possible. 

Depth  of  tunnel 

The  rock  formations  in  general  at  the  required  depths  are  no 
more  objectionable  on  Manhattan  island  or  in  The  Bronx  than  at 
other  localities  on  the  Southern  aqueduct.  There  are  weak  places 
and  crush  zones  to  be  crossed  and  some  of  them  can  not  be  avoided 
by  any  possible  manipulation  of  the  line,  but  these  most  question- 
able spots  constitute  but  a  small  proportion  of  the  whole  distance. 
The  depth  most  suitable  must  depend  chiefly  upon  the  depth  neces- 
sary at  the  worst  spots. 

Comparative  cost  of  construction  if  lines  are  shifted 

The  question  is  best  answered  by  reference  to  the  geological  map. 
It  will  be  noted  especially  that  the  belts  of  the  different  rock  forma- 
tions are  usually  narrow,  and  that  they  run  nearly  parallel  to  the 
average  direction  of  the  lines.  Therefore  a  shift  of  line  to  no  great 
distance  would  at  many  points  place  it  within  an  entirely  different 
formation.  It  is  also  notable  that  all  of  the  lines  run  along  or  near 
the  contacts  between  formations  for  long  distances.  At  such  points 
a  very  small  shift  would  wholly  change  the  type  of  rock  and  rock 
quality.    Some  shifting  is  desirable. 

In  general  it  may  be  assumed  that  the  limestone  belts  would  be 
easiest  and  cheapest  to  penetrate  wherever  they  are  fairly  substan- 
tial, but  they  undoubtedly  also  contain  the  greater  proportion  of 
weak  and  troublesome  ground  and  must  be  considered  least  desir- 
able from  the  standpoint  of  maintenance  and  durability.  The 
gneisses  are  probably  most  expensive  to  penetrate  and  the  schists, 
medium.  Both  are  more  expensive  than  limestone  but  both  arc 
more  likely  to  prove  acceptable  for  other  reasons. 


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NEW  YORK  STATE  MUSEUM 


The  question  of  shifting  the  lines  is  a  complicated  one  and  hinges 
more  upon  rock  conditions,  durability,  and  location  of  weak  zones, 
than  on  any  possible  cost. 

Advisable  changes  in  lines 

None  of  the  suggested  lines  are  defensible  from  a  geologic  point 
of  view  for"  the  reason  that  a  much  better  one  may  be  obtained  by 
no  very  serious  shifting. 

J n  the  general  consideration  of  relative  advantages  of  different 
possible  locations  of  the  line,  it  is  believed  that  the  following  large 
features  are  of  most  immediate  importance: 

1  The  ridges  as  opposed  to  the  valleys. 

2  The  hard  formations  as  opposed  to  the  softer  ones. 

3  The  crossing  of  few  contacts  as  opposed  to  crossing  many. 

4  The  location  well  within  a  formation  as  opposed  to  location 
along  a  contact  zone. 

It  is  distinctly  preferable  from  a  geologic  standpoint  ( I )  to  fol- 
low the  ridges,  (2)  to  keep  in  the  hard  formations,  (3)  to  avoid 
many  changes  from  one  formation  to  another,  (4)  to  keep  away 
from  contact  zones,  and  (5)  to  avoid  weak  zones,  if  possible,  or 
cross  known  troublesome  zones  at  the  most  advantageous  point. 

Recommendations  of  new  lines  F,  G,  H,  I 

The  original  lines  A,  B  and  C  are  marked  on  the  map  in  blue 
[pi.  32].  In  addition  several  trial  lines  are  sketched  in  yellow,  any 
one  of  which  would  give  better  geological  conditions  than  any  of 
the  three  original  lines.  The  newly  suggested  trial  lines  differ  from 
each  other  chiefly  in  the  points  at  which  they  cross  the  limestone 
belts  and  weak  zones.  In  all  of  the  n  the  central  idea  has  been  to 
follow  the  gneiss  and  schist  ridges  as  persistently  as  possible.  All 
unite  at  Central  Park  and  are  intended  to  follow  Fifth  avenue, 
Broadway,  the  Bowery  and  Market  street  to  Fast  river  along  one  of 
the  original  lines.  North  of  Central  Park  they  differ  from  the  orig- 
inal lines.  The  westerly  one  crosses  the  Harlem  river  at  176th 
street  and  may  be  designated  line  F.  The  easterly  line  may  also  cross 
the  Harlem  river  at  176th  street  and  may  be  designated  line  G;  or 
it  may  continue  southward  and  cross  the  Harlem  at  155th  street. 
It  will  then  join  the  first  one  in  the  vicinity  of  144th  street  and  is 
called  line  II.  The  alternative  easterly  one  which  crosses  the  Har- 
lem at  155th  street  and  follows  Seventh  avenue  to  Central  Park  is 
line  I. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


225 


Details  of  rock  conditions  along  these  lines  are  as  follows : 
Line  F.    (Westerly)  beginning  at  Hill  View  reservoir 

Feet 

7  600  Yonkers  gneiss  —  good  quality 
15000  Fordham  gneiss  —  good  quality 
2  000  Fordham  gneiss  —  probably  2d  grade 

1  200  Harlem  river  crossing  —  partly  limestone  —  3d  grade 
14800  Manhattan  schist  —  good  quality 

1  600  Manhattanville  crossing  —  3d  grade  —  some  limestone 

2  600  Manhattan    schist  —  good    rock  —  through  Morningside 

Park 

800  At  south  end  of  Morningside  Park  —  perhaps  some  lime- 
stone —  2d  grade 
1400  Manhattan  sclhst  —  good  —  to  junction 
12000  Manhattan  schist  —  along  Central  Park  —  good 
20600  To  East  river  —  Manhattan  schist  —  less  known1- — -(fair) 
(2d  grade) 
6000  To  Brooklyn  "unknown"1 


85  600  Line  G 

Feet 

8  400  Yonkers  gneiss  —  good  rock 
17  600  Fordham  gneiss  —  good  rock 

which  brings  it  to  the  Harlem  river  where  the  other  line  (F)  is 
joined.  Although  the  line  is  about  1400  feet  longer,  it  avoids  some 
low  ground  (2000  feet)  along  the  east  bank  of  the  Harlem  river, 
some  of  which  may  be  in  poor  condition.  Total  length  of  line, 
87,000  feet. 

Line  H 

Feet 

8  400  Yonkers  gneiss  —  good  quality 
23  800  Fordham  gneiss  —  good  quality  —  to  Harlem  river 
1  000  Crossing  Harlem  river  —  probably  fault  zone  in  gneiss 

800  Fordham  gneiss  —  good  quality 
i  000  Limestone  —  2d  grade 

1  200  Manhattan  schist  —  good  quality  —  to  junction  with  the  first 
line  (F)  at  145th  street 

From  this  point  the  line  is  the  same  as  F  and  G.    Its  chief  ad- 
vantage is  the  great  distance  which  it  has  in  Fordham  gneiss. 
Total  length  of  line,  85,600  feet. 

1  Subsequent  explorations  made  by  the  Board  of  Water  Supply  have  climi-  . 
nated  this  unknown  ground.    See  later  discussion. 


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NEW  YORK  STATE  MUSEUM 


Line  I 

Feet 

8400  Yonkers  gneiss  —  good  quality 
23800  Fordham  gneiss  —  good  quality  —  to  Harlem  river 

1  000  Crossing  Harlem  river  —  probably  fault  zone  in  gneiss 
4400  Fordham  gneiss  —  good  rock  —  to  135th  street 

4  600  Inwood  limestone  —  probably  fair  —  2d  grade 

2  000  Inwood  limestone  —  probably  poor  quality  —  3d  grade 
I  000  Manhattan  schist  —  good  quality 


At  this  point  the  line  unites  with  line  F.  Total  length  of  line, 
83,800  feet. 

A  tabulation  of  these  figures  indicating  estimated  extent  of  rock 
types  is  given  below: 

Line  f     line  g     line  h     line  i 
Feet        Feet         Feet  Feet 

Total  length  of  line   85600  87000  85600  83800 

Length  in  Yonkers  gneiss   7600  8400  8400  8400 

Length  in  Fordham  gneiss   17000  17600  25600  29200 

Length  in  Inwood  limestone  and  marginal 

contacts   3600  3600  3400  6600 

Length  in  Manhattan  schist   51  400  51400  42200  33600 


Comparative  summary  of  types  of  formation  (Comparative  dis- 
tances are  expressed  in  percentages) 


ABC  F  G  h  1 

Yonkers  gneiss   7.2  9.4  7.3  8.8  9.6  9.8  10. o 

Fordham  gneiss    8.1  15.3  26.3  19.8  20.2  29.9  34.8 

Contact  zones   5.1  0.0 

Inwood  limestone    13.9  25.3 

Manhattan  schist    58.5  42.6  7.8  60.0  59.0  49.3  40.1 

Ravenswood  grano-diorite1    0.0  0.0  12.0  0.0  0.0  0.0  0.0 

Too  little  known  to  classify1   7.0  7.0  22.6  7.0  6.9  7.0  7.1 


15.2}  4,2     4,1     39     7  8 


1  The  Ravenswood  granodiorite  has  been  proven  by  later  explorations  to 
extend  into  the  territory  here  marked  as  too  little  known  to  classify. 


As  a  group  it  is  especially  noticeable  that  the  new  lines  F,  G,  H, 
I,  have  a  very  much  lower  percentage  of  contact  zones  and  lime- 
stone. The  percentages  of  gneisses  have  been  notably  increased, 
and  the  unknown  and  questionable  formations  have  been  reduced 
to  approximately  the  lowest  terms. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


22/ 


Estimated  summary  of  quality 


LINE  F           LINE  G 

Feet  Feet 

LINE  H 

Feet 

LINE  I 

Feet 

Good  rock,  first  grade  . . 

  53  40O        56  800 

54600 

49  600 

Fair           second            . . 

01  a  nr\          0  t  a  nr\ 

O  ~>  4  OO 

Poor      "    third  " 

  2  800         •  2  800 

2  600 

3  000 

I   n  L' ti  ( \w  n  1     (  Rrriol<7 1  vn  ^ 

V_  1 1 1\.  1 1  U  \\  1 1         \  J_>  1  UUMJ  11  i        .  . 

fi  000          fs  noo 

6  000 

6  000 

85  600     87  000 

85  600 

83800 

1  All  of  this  rock  is  now 

known  to  be  of  good  quality. 

In  other  words  these 

new  lines  show : 

LINE   F        LINE  G 

Per  cent    Per  cent 

LINE  H 

Per  cent 

LINE  I 
Per  cent 

•  •                                  62.3  65.3 

63.8 

59- 1 

Second   

  27.3  24 

.6 

26. 1 

30.0 

9 

89.9 

89.  i 

  3-2  3 

0 

3-0 

3-6 

"  Unknown  "  ground  1  .... 

  7.0  6 

9 

7.0 

7-i 

1  Results  of  recent  boring  explorations  show  that  this  ground  is  first 
grade  also. 

A  comparison  on  this  basis  with  the  original  lines  A,  B,  C  indi- 
cates that  these  new  lines  F,  G,  H,  I,  make  a  better  showing, 
especially  on  first  grade  rock  and  that  all  show  decided  reduction  in 
the  third  grade  ground. 


ABC         F  G        h  1 

First  grade  rock                            49.4    40.0    49.0  62.3  65.3  63.8  59.1 

Second  grade  rock                        35.9   41. 1    16.7  27.3  24.6  26.1  30.0 

First  and  second                          85.3   81.0   65.7  89.6  89.9  89.9  89.1 

Third  grade  rock                            7.7    11. 8    11. 6     3.2  3.0     3.0  3.6 

Unknown 1                                     7.0     7.1    22. 7     7.0  6.9     7.0  7.1 


1  Now  known  to  be  first  grade. 

On  geological  grounds,  therefore,  it  is  confidently  believed  that 
any  one  of  the  new  lines  (F,  G,  H,  I)  would  give  decidedly  better 
results  than  any  one  of  the  original  ones  (A,  B,  C).  The  poor 
and  the  questionable  and  the  unknown  ground  can  not  be  wholly 
avoided  by  any  possible  line,  no  matter  how  roundabout,  in  these 
lines,  approximately  as  drawn,  the  objectionable  points  are  reduced 
to  a  minimum  with  almost  no  increase  in  total  length  of  conduit. 
The  objectionable  portions  are  also  restricted  in  large  part  to  the 
8 


228 


NEW  YORK  STATE  MUSEUM 


Harlem  river,  where  we  already  have  the  experience  of  the  last 
aqueduct  (the  New  Croton  aqueduct)  as  a  guide,  and  a  very  few 
other  spots. 

General  conclusions 

Line  I  is  the  shortest  possible  defensible  line.  Its  chief  objec- 
tionable feature  is  a  rather  long  stretch,  6600  feet  of  limestone, 
from  135th  street  to  Central  Park,  upon  the  quality  of  which  there 
are  no  data.  It  crosses  the  Harlem  river  fault  probably  in  gneiss. 
But  it  crosses  the  extension  of  the  Manhattanville  fault  in  lime- 
stone. 

Lines  F,  G  and  H  are  almost  equally  defensible.  Line  G  is 
longest,  but  is  in  some  respects  —  especially  in  following  the  ridge 
crests  —  one  of  the  best  possible  locations. 

It  should  be  appreciated  that  many  other  matters,  such  as 
municipal  works  already  completed  or  projected,  or  matters  of 
engineering  practice,  are  likely  to  make  it  necessary  to  modify  any 
line  proposed,  and  that  the  final  line  is  more  likely  to  be  a  com- 
promise, considering  all  interests. 

A  graphic  representation  of  the  comparative  merits  of  the  pro- 
posed lines  is  given  in  plate  33.  This  is  strictly  a  geologic  study. 
The  lines  are  properly  placed  on  an  outline  map  of  the  city  corre- 
sponding exactly  to  those  drawn  on  the  geologic  map,  plate  32. 
The  geologic  formations  that  each  would  cut  are  represented  on 
longitudinal  sections  which  follow  each  line,  and  the  attitude  and 
structure  of  each  formation  are  indicated. 

Revised  lines 

Subsequently  two  revised  lines  based  upon  the  preceding  studies 
were  examined  to  determine  preference.  Later  one  of  these,  or  a 
slight  modification  of  it,  was  adopted  as  the  one  to  be  explored. 
It  was  soon  determined  on  the  same  reasoning  as  was  applied  to 
the  first  group  of  lines  that  the  most  westerly  line  —  the  line  keep- 
ing as  much  as  possible  within  the  gneiss  and  schist  ridges  — 
would  be  the  most  likely  to  give  satisfactory  conditions.  By  this 
method  of  selection  the  unknown  or  untested  and  doubtful  ground 
was  reduced  to  its  lowest  limits.  It  was  found  that  nearly  all  of 
the  very  weak  spots  could  be  located  by  inspection  in  the  northern 
portion  of  the  line,  but  south  of  59th  street  the  question  is  de- 
cidedly more  difficult  because  of  the  heavy  drift  cover.  No  rock 
outcrops  occur  south  of  30th  street,  and  one  is  reduced  to  the 
evidence  of  deep  borings. 


Geologic  detail  of  the  Manhattanville-Morningside  section  showing  the  alternative  lines  studied,  the  locations  of  exploratory  borings,  the  two 

principal  crush  zones  and  longitudinal  profiles 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


229 


Points  for  exploration  north  of  59th  street 

It  was  soon  evident  that  extensive  exploratory  work  would  have 
to  be  undertaken  and  the  following  points  were  selected  at  which  to 
begin. 

1  The  Harlem  river  crossing,  where  the  distribution  conduit  line 
crosses  the  river  just  below  High  Bridge  [see  later  description]. 
The  only  good  evidence  as  to  character  of  rock  at  this  place  is  from 
the  pressure  tunnel  of  the  New  Croton  aqueduct  which  crosses  the 
river  a  short  distance  above. 

2  The  Manhattanville  cross  valley  (125th  street  depression). 
This  is  the  most  important  cross  depression  on  the  island  of  Man- 
hattan. It  is  apparent  after  a  little  investigation  that  the  bed  rock 
floor  lies  deep  and  that  if  it  were  not  for  the  drift  filling  the  tides 
would  surge  through  this  valley  making  a  direct  connection  between 
the  Hudson  and  East  river.  It  was  the  least  known  as  to  depth 
and  character  of  any  point  along  the  proposed  line. 

3  The  depression  between  Morningside  and  Central  Park.  At 
that  place  limestone  on  the  crest  of  a  pitching  anticline  reaches 
farther  south  than  on  either  side  and  is  more  deeply  eroded.  The 
other  zones  of  large  importance  are  in  southern  Manhattan  the 
geology  of  which  is  a  special  study. 


CHAPTER  XV III 


AREAL    AND    STRUCTURAL    GEOLOGY    SOUTH    OF  59TH 

STREET 

The  necessity  for  exploration  in  certain  sections  of  this  area  can 
not  be  appreciated  without  a  statement  of  the  local  geology  and 
especially  of  the  revision  of  both  areal  and  structural  geology  that 
the  writer  has  based  upon  an  exhaustive  study  of  all  the  available 
drill  cores  and  other  data  to  be  found  in  southern  Manhattan,  East 
river  and  Brooklyn. 

Below  Central  Park  there  is  now  little  geology  to  be  gathered 
from  a  study  of  the  present  surface.  But  as  far  south  as  31st 
street  the  bed  rock  geology  is  pretty  well  known  from  earlier  re- 
ports and  from  recent  improvements  that  have  exposed  the  under- 
lying rock.  All  of  this  portion  is  mapped  as  Manhattan  schist  ex- 
cept one  small  area  of  serpentine  at  59th  street  between  10th  and 
nth  avenues.  There  is  no  reason  to  modify  this  usage.  A  careful 
study  of  a  great  number  of  rock  borings  from  the  Pennsylvania 
Railroad  tunnel  across  Manhattan  at  32d  street  proves  beyond 
question  that  bed  rock  is  Manhattan  schist,  including  almost  all 
known  variations  and  accompaniments,  for  the  whole  width  of  the 
island  along  that  line. 

Still  farther  southward  the  points  that  have  yielded  exact  in- 
formation about  bed  rock  are  less  numerous,  and  below  14th  street 
are  confined  to  deep  borings  or  an  occasional  very  deep  excavation 
for  foundations.  Even  these  sources  of  information  are  lacking 
over  large  areas.  The  greater  number  of  borings  available  are 
along  the  water  front.  Their  distribution  is  such  as  to  indicate  that 
the  west  side  and  central  portion  and  southerly  extremity  of  the 
island  are  all  underlain  by  Manhattan  schist.  This  is  true  eastward 
to  the  East  river  at  27th  street,  and  as  far  eastward  as  Tompkins 
square  at  10th  street  and  almost  to  the  Manhattan  tower  of  Brook- 
lyn bridge  in  that  vicinity. 

To  the  eastward  of  these  limits,  i.  e.  to  the  eastward  of  the  line 
projected  from  Blackwell's  Island  to  the  Manhattan  tower  of 
Brooklyn  bridge,  there  is  a  more  complicated  geology.  The  borings 
of  the  East  river  water  front  are  decidedly  variable.  They  are 
certainly  not  all  Manhattan  schist  of  the  usual  types.  Those  most 
unlike  the  Manhattan  are  at  the  same  time  most  like  some  varieties 

231 


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NEW  YORK  STATE  MUSEUM 


of  the  Fordham,  and  indicate  that  these  formations  both  occur. 
The  lack  of  any  data  in  the  beginning  of  this  investigation  except 
on  the  water  front  made  it  impossible  to  draw  more  than  very  gen- 
eral lines.  Drawn  in  this  way,  the  lines  of  course  are  too  straight, 
but  it  is  certain  that  they  indicate  more  nearly  the  actual  existing 
areal  distribution  of  formations  than  any  of  the  maps  now  in  exist- 
ence.1 They  indicate  a  southward  extension  of  the  Blackwell's 
Island  belt  of  Fordham  gneiss  toward  the  Manhattan  tower  of 
Brooklyn  bridge.  How  much  of  this  anticlinal  fold  of  Fordham 
actually  brings  this  formation  to  the  surface  it  is  impossible  to  say, 
but  that  it  may  be  expected  to  be  encountered  along  this  line  is 
evident. 

On  the  east  side  a  parallel  belt  of  Inwood  limestone  is  indi- 
cated and  this  again  is  succeeded  by  a  Fordham  gneiss  area 
which  occupies  the  rest  of  the  eastern  margin.  Explorations 
made  along  the  line  of  the  gas  tunnel  across  East  river  at  72d 
street2  indicates  comparatively  narrow  belts  of  limestone  there 
in  both  the  east  and  west  channels.  The  limited  width  of  limestone 
at  these  points,  together  with  the  occurrence  of  two  strongly  de- 
veloped disintegration  zones,  seem  to  indicate  rather  extensive 
squeezing  out  and  faulting  of  this  formation  along  fault  planes 

1  In  the  summer  of  1908  the  writer  was  assigned  the  task  of  studying 
in  detail  the  evidences  of  geologic  structure  beneath  the  drift  in  southern 
Manhattan.  Before  any  drilling  was  attempted  in  the  city  by  the  Board 
of  Water  Supply,  a  thorough  canvass  was  made  of  all  previous  borings  in 
this  district  and  the  cores  and  records  were  personally  inspected.  More 
than  300  such  borings  were  found  in  which  some  of  the  core  could  be 
secured  for  identification  and  classification  as  to  formation  and  condition. 
Most  borings  were  given  no  weight  at  all  in  the  final  summary  of  this 
evidence  unless  the  rock  core  or  at  least  fragments  of  it  could  be  secured. 
After  all  of  these  newly  assembled  data  were  tabulated  and  plotted  on  the 
map,  it  was  evident  that  if  the  identifications  were  correct  the  areal  and 
structural  map  of  southern  Manhattan  needed  extensive  revision.  A  new 
map  therefore  was  made  and  presented  to  the  chief  engineer  of  the  Board, 
October  30,  1908.  This  has  been  used  since  as  the  basis  for  exploration  of 
the  Lower  East  Side  section.  This  original  tabulation  and  map  only 
slightly  modified  was  published  under  the  Areal  and  Structural  Geology  of 
Southern  Manhattan  Island  [N.  Y.  Acad.  Sci.  Annals,  April  1910,  v.  19,  no. 
11,  pt  2].  The  extensive  explorations  of  the  board  have  made  further  revision 
necessary  [see  accompanying  map,  pi.  34].  Exploratory  boring  is  still  in 
progress  (October  1910)  and  some  slight  modifications  of  boundary  lines 
may  yet  be  made. 

2  This  is  taken  from  Prof.  J.  F.  Kemp's  description  of  The  Geologic  Sec- 
tion of  the  East  River  at  Seventieth  Street,  New  York  [N.  Y.  Acad.  Sci. 
Trans.  1895.  14:273-76]. 


N.  Y.  State  Museum  Bulletin  146 


Plate  34 


e  map  reprtfdurtHl  frt 


5  Beekman  street,  and  ber*  used  by  permlssl 


Margin-  of  Long  Island 


This  revision 


Revised  Areal  Geology  op  Southern  Manhattan  Island  and  the  Adjacent 
is  based  upon  exploratory  borings  to  June  25th.  1910.    The  heavy  blue  line  marks  the  course  of  the  proposed  pressure  tunnel  intended  to  carry 

the  Catskill  water  to  Brooklyn 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


233 


parallel  to  the  strike.  Such  movements  are  capable  of  cutting  out 
the  intermediate  limestone  entirely  from  between  the  schist  and 
gneiss.  How  much  of  such  modification  exists,  in  the  almost  total 
lack  of  data  bearing  upon  the  question,  it  is  impossible  to  say.  The 
intermediate  belt  is  indicated  on  the  accompanying  map  [pi.  34],  as  a 
limestone  area.  At  one  point  at  least  the  limestone  does  occur  in 
the  older  borings,  i.  e.  on  the  southeastern  margin  of  the  Man- 
hattan pier  of  the  Manhattan  bridge  (bridge  no.  3),  at  the  foot  of 
Pike  street. 

On  the  Brooklyn  side  no  formations  of  this  series  except  the 
Fordham  and  its  associated  igneous  masses,  such  as  the  Ravens- 
wood  granodiorite,  have  been  identified  within  the  area  under  study. 
Limestone  is  reported  (Hobbs  reference  to  Veatch)  near  Newtown 
creek,  a  little  beyond  the  eastern  margin  of  the  present  map. 

Structure  of  the  East  river  area 
Manhattan  side.     In  all  of  the  area  south  of  59th  street, 
structural  features  are  even  more  obscure  than  the  areal  geology. 

There  is  no  reasonable  doubt  but  that  weak  zones  will  be  found 
as  frequently  in  the  Manhattan  schist  portion  of  this  area  as  on  the 
line  north  of  59th  street,  but  they  can  not  be  indicated  as  closely. 
No  cross  fault  of  large  consequence  can  be  identified,  but  there  is 
some  evidence  of  a  minor  zone  that  should  be  encountered  on  Fifth 
avenue,  in  the  vicinity  of  32d  street.  The  Pennsylvania  tunnels  and 
the  subway  both  cross  this  line  and  so  far  as  known  there  were  no 
serious  weaknesses  developed.  There  is  nowhere  any  evidence  of 
an  important  depression  like  the  Manhattanville  valley. 

It  is  confidently  believed  that  the  problems  on  this  southerly  por- 
tion of  Manhattan  are  involved  chiefly  with  the  longitudinal  struc- 
tures produced  by  folding  and  faulting  and  subsequent  disintegration 
along  such  zones. 

Crossing  of  East  river 

From  59th  street  to  the  East  river  there  seems  to  be  no  reason 
for  a  preference  between  the  two  lines  P  and  Q.1  On  the  Brooklyn 
side  likewise  there  is  no  known  geological  reason  for  preference. 
Such  basis  for  choice  as  is  now  known  relates  to  the  East  river 
channel  alone.  Since  this  is  at  the  same  time  the  most  difficult  sec- 
tion of  the  line  to  explore  and  probably  the  most  uncertain  section 
to  estimate  as  to  condition  and  consequent  depth  of  tunnel,  it  would 
be  especially  useful  to  be  able  to  make  a  decisive  selection  of  cross- 
ings at  once. 


1  For  location  of  these  lines  sec  map,  pi.  32. 


234 


NEW  YORK  STATE  MUSEUM 


Such  evidence  as  has  any  bearing  upon  this  question  has  already 
been  used  in  formulating  the  interpretation  of  geologic  structure 
given  in  the  foregoing  sections  of  this  report.  If  the  succession  and 
boundaries  of  formations  as  outlined  are  reasonably  close  to  the 
actual  conditions,  it  would  appear  that  line  P  (the  southerly  one 
just  above  Manhattan  bridge )  lias  some  advantage  over  line  Q 
(near  Williamsburg  bridge).  The  chief  elements  in  this  advantage 
are  as  follows : 

1  It  would  appear  that  line  P  might  lie  wholly  within  the  Ford- 
ham  gneiss  in  the  East  river  section,  while  line  Q  may  cross  two 
contacts. 

2  From  the  evidence  of  borings  made  in  the  East  river  at  14th 
street1  it  appears  probable  that  a  belt  of  schist  similar  to  Manhattan 
schist  in  quality  (whether  accompanied  by  limestone  or  not  there  is 
no  direct  evidence)  lies  in  the  river  channel  toward  the  east  side  and 
in  all  probability  extends  southward  in  the  middle  of  the  river  at 
Williamsburg  bridge.  This  would  be  cut  by  line  Q.  The  uncertain- 
ties of  this  association  are  of  sufficient  importance  to  throw  the  bal- 
ance of  present  choice  toward  line  P. 

3  If  the  theory  that  the  East  river  course  is  due  chiefly  to  zones 
of  weakness  following  fractures  or  faults  is  true,  their  possible 
comparative  condition  as  they  cut  through  different  formations  must 
be  taken  into  account.  There  is  little  doubt  on  this  point  but  that, 
in  zones  of  similar  original  disturbance,  those  in  the  Fordham 
gneiss  have  suffered  less  extensively  from  disintegration  than  those 
cutting  either  the  limestone  or  schist.  Therefore,  obscure  as  it  may 
be,  the  preference  is  again  in  favor  of  line  P. 

4  If,  furthermore,  the  course  of  the  river  is  due  to  cross  faulting 
or  any  similar  or  related  displacements  or  movements,  an  inspection 
of  the  structural  map  indicates  that  the  controlling  zone  followed 
by  the  river  as  it  crosses  line  O  must  have  a  general  strike  north- 
west, while  the  corresponding  zone  that  crosses  line  P  strikes  east. 
Of  these  two  types  (directions)  of  fault  zones,  so  far  as  they  may 
be  judged  to  have  influence  in  the  adjacent  area,  there  is  no  doubt 
but  that  the  northwest  type  (the  set  that  has  a  northwest  strike) 
is  both  the  more  common  and  the  more  important.  If  this  general 
tendency  is  also  true  here,  then  on  this  account  also  line  P  may  be 
considered  slightly  more  favored.    In  reality  not  much  weight  can 

1  These  borings  were  made  by  the  Public  Service  Commission  in  explora- 
tions for  subways. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  235 

be  given  to  this  point  since  the  condition  of  these  faults  is  not  fully 
known. 

5  If,  as  may  well  happen,  the  present  East  river  is  displaced1  from 
its  old  channel  by  glacial  drift,  so  that  it  is  essentially  an  evicted 
stream,  there  may  not  be  as  pronounced  a  channel  or  as  weak  ground 
to  cross  at  such  points  as  at  those  where  the  old  channel  is  still  oc- 
cupied.  In  such  case  both  of  these  lines  are  favorable. 

6  On  the  other  hand,  the  crossing  of  line  P  is  almost  a  mile 
nearer  to  the  great  Hudson  gorges,  to  which  doubtless  this  portion 
of  the  preglacial  East  river  was  tributary,  and  consequently  its  bed 
rock  channel,  if  it  is  the  real  preglacial  channel,  may  be  expected 
to  be  deeper  and  the  accompanying  disintegration  (so  far  as  it  may 
be  controlled  by  this  factor)  may  be  expected  to  reach  lower  than  at 
points  in  similar  surroundings  farther  up  stream.  It  is  impossible 
to  say  how  much  weight  should  be  given  to  this  objection.  It  does 
not  seem  to  be  of  sufficient  importance  to  fully  offset  the  favorable 
features  indicated  in  items  1,  2,  3  and  4. 

On  the  basis  of  these  studies  line  P  (the  southerly  one)  near 
Manhattan  bridge  was  chosen  as  the  site  of  preliminary  exploration 
promising  the  most  favorable  results.  Eater  this  was  shifted  a  short 
distance  without  introducing  any  new  conditions. 

1  Exploratory  borings  indicate  that  such  has  been  the  history  of  the  river. 


CHAPTER  XIX 


SPECIAL   EXPLORATION  ZONES 

Exploration  by  borings1  and  other  methods  have  been  made  at  all 
questionable  or  uncertain  points  along  the  line.  As  was  expected 
in  the  beginning  five  places  have  required  elaborate  exploration  and 
some  exceptional  conditions  have  been  proven.  The  original 
geological  investigation  based  upon  surface  study  as  outlined  in  the 
foregoing  pages  served  to  locate  these  spots  accurately. 

These  places  or  zones,  now  either  finished  or  sufficiently  well 
known  to  permit  accurate  statement  of  geologic  conditions,  are  as 
follows : 

1  The  Harlem  river  crossing  at  167th  street,  where  the  aqueduct 
will  cross  from  a  ridge  of  Fordham  gneiss  beneath  the  Harlem  river, 
where  the  whole  thickness  of  Inwood  limestone  will  be  cut,  to  the 
ridge  of  Manhattan  schist  above  the  Speedway  on  Manhattan  island. 

2  The  Manhattanville  cross  valley,  a  low  pass  crossing  the  island 
at  about  125th  street.  The  part  explored  extends  from  St  Nicholas 
to  Morningside  Parks  and  crosses  a  zone  with  very  low  rock  floor 
in  the  Manhattan  schist. 

3  From  Morningside  to  Central  Parks.  The  line  crosses  the 
strike  of  the  formations  at  this  point  and  cuts  a  longitudinal  fault 
and  anticlinal  fold  which  tends  to  bring  the  Inwood  limestone 
within  surface  influence. 


1  Exploratory  work  has  been  in  direct  charge  of  Mr  T.  C.  Atwood,  division 
engineer,  who  has  followed  all  stages  of  it  almost  from  the  beginning.  In 
the  later  exploratory  work  an  immense  amount  of  detail  and  a  very  com- 
plex lot  of  data  has  accumulated  requiring  constantly  the  services  of  a  man 
with  some  special  geological  training.  Mr  John  R.  Healey,  formerly  in  the 
testing  laboratory,  was  transferred  to  this  special  field.  He  is  probably 
more  familiar  with  the  multitude  of  details  resulting  from  boring  operations 
along  the  conduit  line  than  any  one  else.  Except  for  the  care  and  good 
judgment  used  by  these  men  in  preserving  data,  and  the  wisdom  of  the 
men  who  planned  the  line  and  methods  of  work  before  them,  much  valu- 
able geologic  data  would  have  been  lost.  Notwithstanding  the  best  efforts 
of  the  consulting  geologist  some  really  critical  points  escape  unless  some  one 
constantly  on  the  ground  is  directly  interested  in  them  as  a  part  of  the 
regular  responsibility. 

237 


238 


NEW  YORK  STATE  MUSEUM 


4  The  Lower  East  Side  zone.  On  Delancey  street  east  of  the 
Bowery,  the  line  crosses  the  structure  and  at  this  point  the  whole 
series  of  crystalline  formations  appears.  Besides  complicated  struc- 
ture there  is  also  exceptionally  deep  alternation  or  decay  of  bed  rock. 

5  The  East  river  crossing — from  the  foot  of  Clinton  street  to 
Bridge  street,  Brooklyn. 

i  Harlem  river  crossing 

Geologically  the  Harlem  river  between  155th  and  200th  streets 
has  the  same  relation  to  local  formations  for  the  whole  distance. 
It  flows  on  the  Inwood  limestone  bed  which  stands  almost  exactly  on 
edge,  while  the  east  river-bluff  is  formed  by  the  underlying  Fordham 
gneiss,  and  the  west,  by  a  strong  escarpment  of  Manhattan  schist 
which  extends  southward  throughout  the  whole  of  Manhattan  form- 
ing the  backbone  of  the  island. 

At  the  selected  crossing  a  short  distance  below  High  Bridge,  near 
167th  street,  the  schist-limestone  contact  is  in  the  river  and  appears 
to  be  a  low  weak  spot  [see  detail  of  record].  The  limestone-gneiss 
contact  however  is  in  the  flat  east  of  the  river  bank,  near  Sedgwick 
avenue  and  seems  to  be  more  substantial.  The  structural  detail  and 
relations  are  shown  on  the  accompanying  profile  and  cross  section, 
[Pi.  35]. 

It  is  observed  by  examination  of  the  data  secured  by  borings  that 
the  limestone  formation  at  this  point  is  exceptionally  heavily  im- 
pregnated with  pegmatite  dikes  and  stringers,  and  that  interbedded 
schist  layers  are  large  and  numerous. 

The  weakest  spot  found  lies  at  the  contact  between  schist  and 
limestone  where  there  is  probably  some  longitudinal  displacement. 

A  similar  condition  was  found  at  the  new  Croton  crossing  2000 
feet  farther  north.  On  the  whole  bad  decay  does  not  extend  very 
deep —  150-200  feet. 

Several  borings  have  been  made  and  on  them  is  based  the  only 
judgment  possible  of  the  actual  structure  and  physical  condition  of 
rock.  In  most  cases  the  evidence  is  easily  interpreted  for  these 
points.  The  most  weakened  spot,  as  well  as  the  most  difficult  to 
interpret  in  all  its  detail,  is  the  limestone-schist  contact.  It  is 
judged  that  hole  no.  17  cut  through  this  contact  zone.  This  boring 
is  located  in  the  river  50  feet  from  the  Speedway  (west  bank)  on 
the  proposed  tunnel  line  which  crosses  a  short  distance  south  of 
High  Bridge.    It  is  known  as  hole  no.  17/C38.    Because  of  the 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT  239 

somewhat  unusual  quality  of  material  at  this  place  as  indicated  by 
the  wash  and  core  saved  and  because  of  the  suggestion  it  gives 


Fig.  38  Key  map  showing  plan  of  exploratory  borings  at  the  Harlem 
river  crossing,  location  of  the  New  Croton  aqueduct  which  crosses  the 
Harlem  in  a  pressure  tunnel  and  the  Old  Croton  aqueduct  which  crosses  the 
river  on  High  Bridge 


about  the  structure  and  condition  of  rock  beneath  the  river,  the 
record  and  interpretation  notes  are  given. 


240 


NEW  YORK   STATE  MUSEUM 


Feet 

o —  i3=Water 

13 —  46=Black  river  mud  (mostly  river  silt) 
46 —  48=Sand  with  decayed  wood  (peaty  wood) 
48 —  7o=Quartz  and  garnet  sand  rather  clean  (glacial) 
46 —  70=Lumps  of  peaty  matter  coming  to  the  surface  at  inter- 
vals   indicating    occasional    small    layers    of  peat 
(glacial) 
70 —  78=Mixed  sand  (glacial) 

92  =A  core  of  Triassic  contact  shale  (a  drift  boulder  from 
the  Palisade  margin).  At  this  point  also  a  piece  of 
Manhattan  schist  (boulder) 

95  =4  pieces  of  diabase  (Palisade  trap)  from  another  drift 
boulder 

9^-5  =5  pieces  of  Inwood  limestone  (boulder)  followed  by  a 
piece  of  quartzite  and  several  mixed  pebbles  indi- 
cating glacial  drift  origin 

114 — II9=A  buff  yellow  sand  with  much  pearly  yellow  mica  flakes. 

Effervesces  with  acid.  This  shows  no  foreign  matter. 
It  is  chiefly  residuary  decayed  rock  in  place  and  repre- 
sents silicious  and  micaceous  limestone.  It  is  decayed, 
very  impure,  Inwood  limestone 

119  =Clay  with  pieces  of  flinty  quartzite,  probably  from  a 
small  quartzose  seam  in  the  limestone 

120 — i2(5=Light  flaky  yellow  material.  Much  pearly  mica  with 
earthy  matter.  Effervesces  in  acid.  Residuary  from 
Inwood  limestone 

128  =White  and  drab  lumpy  residuary  matter  (kaolin)  and 
earthy  substances.  Effervesces.  A  more  impure  In- 
wood. Also  shows  several  pieces  of  core  of  a  porous, 
rotten  limestone.  Inwood 

129 — i34=Reddish    brown   lumps.     Effervesces    a    very  little. 

Mostly  clay  but  still  no  foreign  matter.  Residuary 
material  from  a  more  silicious  bed.  A  few  pieces  of 
hard,  impure  limestone  at  133  feet 

134         =Pieces  of  a  porous  quartz  chlorite  rock  with'  little  lime. 

Is  a  leached  quartzose  rock  evidently  a  sandstone 
layer  in  the  limestone.  Rock  belongs  to  the  Inwood 
formation 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


24I 


Feet 

135 — i43=Dark  micaceous  matter  containing  chiefly   biotite,  a 

pearly  mica,  and  quartz.    Rock  is  a  decayed  schist 

bed=the  transition  between  Inwood  limestone  and 

Manhattan  schist 
143 — i5i=Dark  brown  micaceous  material.    Biotite  and  quartz  — 

chiefly.    Rock  is  decayed   schist    (transition  rock). 

At  146  feet  encountered  pieces  of  a  pegmatite  veinlet. 

All  pieces  except  1  are  pegmatitic  —  the  other  one  is 

calcareous  sandstone,  fallen  into  this  lot  from  the 

134  foot  level 
151 — i6o=Chunks  of  pegmatite  (a  vein  rock) 
151 — i6i=The  mica  washings  continue  the  same  as  at  143 — 151 

feet.     Rock  is  a  transition  schist  with  pegmatite 

stringers 

164 — i6o==Brownish  yellow  micaceous  matter  (loose).  Mica, 
quartz,  chlorite,  lime.  Effervesces 

164 — i73=Many  pieces  of  typical  Manhattan  schist.  A  fair 
amount  of  core  for  the  conditions.  Rock  is  not  so 
badly  decayed  but  is  broken  into  small  pieces.  Rock 
is  Manhattan  schist  of  typical  character. 

Summary 

1  The  material  is  chiefly  river  silt  down  to  46  feet 

2  Lighter  glacial  deposits  46 — 78  feet 

3  Heavy  bouldery  drift  78 — 97  feet 

4  Uncertain  (insufficient  data)  97 — 114  feet 

5  Residuary  micaceous  decay  products  from  Inwood  limestone 

114— 135  feet 

6  Decayed  transition  schist  bed  with  some  lime,  but  chiefly  like  the 

Manhattan  schist  135 — 161  feet 

7  More  calcareous  schist  161 — 164  feet 

8  Typical  Manhattan  schist  164 — 173  feet 

Interpretation 

1  Foreign  matters,  glacial  and  recent  deposits,  continue  to  a  depth 

of  between  97  and  114  feet. 

2  Rotten  formations  (residuary  matter)  in  place  begin  at  least  as 

high  as  114  feet.  There  is  no  foreign  material  below  that  point 
except  grains  that  have  fallen  into  the  hole  from  above. 


242 


NEW  YORK  STATE  MUSEUM 


3  More  solid  rock  begins  at  164  feet. 

4  The  upper  portion  of  the  rotten  rock  (114-35  feet)  is  calcareous 

enough  to  belong  to  the  Inwood  limestone  formation.  The 
lower  9  feet  (164-73  feet)  is  typical  Manhattan  schist.  The 
intermediate  ground  135-64  feet  is  transition  variety. 

5  The  drill  has  cut  the  contact  between  Inwood  and  Manhattan 

formation. 

6  If  this  identification  of  the  badly  decayed  matter  is  correct,  the 

contact  at  this  point  dips  steeply  eastward,  i.  e.  it  is  overturned. 

7  Both  types  of  rock  are  shown  to  be  extensively  decayed. 

8  The  worst  (deepest)  decay  zone  probably  lies  still  a  little  farther 

east,  and  follows  the  dip  of  the  micaceous  limestone  near  the 
contact. 

These  conditions  are  indicated  on  the  accompanying  cross  section 
[see  pi.  35]. 

The  conditions  indicated  by  this  one  hole  are  consistent  with 
those  known  for  the  New  Croton  aqueduct  tunnel  2000  feet  farther 
north  where,  according  to  the  engineers'  drawings,  the  formations 
also  are  overturned.  Fifty  feet  of  decayed  rock  is  shown  in  this 
hole.  The  contact  is  undoubtedly  decayed  considerably  to  a  depth 
of  more  than  200  feet  below  water  level. 


Fig.  39    Harlem  river  crossing  —  New  Croton  aqueduct 


Another  boring  put  down  to  test  conditions  at  still  greater  depth 
nearby  explored  the  rock  to -442.7  feet.  Because  of  the  informa- 
tion it  gives  about  the  deeper  bed  rock,  a  summary  of  the  record 
based  upon  examination  of  the  material  is  given : 


Geologic  cross  section  and  graphic  interpretation  of  the  exploratory  borings  made  for  the  New  York  City  Board  of  Water  Supply  at  the  site  of  the  proposed  pressure  tunnel  beneath  the  Harlem 

river,  reaching  Manhattan  at  the  foot  of  171st  street 


EL 
l< 


-  IC 

-2C 
-3C 
-40 


n 
ti 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


243 


Hole  no.  42  (75  feet  from  Speedway,  25  feet  east  of  hole  no.  17) 

Feet 


0 

to 

-94.1 

River  muds  and  various  types  of  drift  similar  to 

hole  no.  17 

-94 

to 

^6 

Iron  cemented  sand  —  both  drift  sand  and  local 

angular  material 

-98 

to 

-127 

Micaceous   clay  —  residuary   decayed   matter  — 

with  choppings  of  calcite,  quartz,  mica  and 

chlorite  representing  weathered  Inwood  lime- 

stone 

-127 

to 

-135 

Core  —  Inwood  limestone  (impure) 

-135 

to 

-H9-5 

Pegmatite 

-H9-5 

to 

-197 

Inwood    limestone  —  typical  —  standing  almost 

vertical  in  upper  portion  but  changing  to  about 

450  and  farther  down  to  60°.    Good  sound 

rock 

-197 

to 

-241 

Manhattan    schist  —  of    typical    sort  —  and  in 

sound  condition,  but  becoming  somewhat  more 

broken  and  altered  near  the  bottom.   Dip  about 

6o°-8o°   and  even  more.     Average  probably 

75°-8o° 

-241 

to 

-295 

Manhattan    schist  —  typical  —  dip    variable  but 

mostly  above  700  to  vertical  —  some  pegmatite 

—  fractures  are  at  high  angle.    Rock  sound 

-295 

to 

-302.5 

Pegmatite 

-302.5 

to 

-442.7 

Inwood  limestone  —  typical  —  good  quality  —  dip 

700  to  very  flat  —  one  piece  not  over  350  but 
mostly  obscure 
An  interpretation  summary  is  as  follows: 

Feet 

o  to    -94    River  muds  and  drift  filling  (glacial  and  recent) 
-94  to    -96     Transition  to  residuary  matter 
-96  to  -127    Residuary  matter  and  badly  decayed  Inwood  lime- 
stone 

-127  to  -197  Inwood  limestone 
-197  to  -302  Manhattan  schist 
-302  to  -442.7  Inwood  limestone 

Geologic  cross  section.  The  accompanying  cross  section 
[pl-  35]  embodies  an  interpretation  of  all  the  data  secured  in  the 
Harlem  river.    It  is  now  known  that  the  limestone  is  overturned 


244 


NEW  YORK  STATE  MUSEUM 


slightly  at  both  contacts.  The  nature  of  these  contacts  makes  it 
seem  probable  that  there  is  very  little  of  the  limestone  squeezed  or 
cut  out  by  movement.  Therefore  this  crossing  gives  a  fairly  ac- 
curate measure  of  the  thickness  of  the  Inwood.  This  is  approxi- 
mately 750  feet.  No  section  about  New  York  city  is  more  accurately 
determined. 

2  Manhattanville  cross  valley 

In  northern  Manhattan  the  schist  ridge  which  forms  the  back- 
bone of  the  island  and  has  a  relief  of  more  than  100  feet,  is  cut 
across  by  a  prominent  valley  that  extends  from  the  Hudson  at 
130th  street  eastward  to  the  Harlem  Flats  and  East  river.  This 
valley  is  nowhere  more  than  25  or  30  feet  above  the  sea  level  and 
is  drift  filled.  Previous  to  the  recent  boring  explorations  of  the 
Board  of  Water  Supply  its  true  depth  to  rock  floor  was  unknown. 
The  few  borings  recorded,  however,  indicated  a  depth  of  more  than 
a  hundred  feet.  One  such  boring  at  129th  street  and  Amsterdam 
avenue  is  reported  as  penetrating  109  feet  from  surface  without 
touching  rock.  Another  of  similar  results  is  located  at  125th  and 
Manhattan  streets  where  a  depth  of  204  feet  failed  to  touch  rock. 

Besides  determining  rock  floor  in  the  present  case,  it  was  im- 
portant to  determine  rock  structure  and  conditions.  It  appears 
from  surface  features  that  this  cross  valley  probably  follows  a 
fault  zone  along  which  there  has  been  weakening  of  the  rock  and 
consequent  disintegration  and  decay.  If  this  is  so  it  would  be  ad- 
vantageous to  find  the  limits  of  it  and  determine  what  displace- 
ment effects  were  produced.  It  has  been  surmised  by  all  students 
of  local  geology  that  such  cross  faults  may  lift  the  blocks  on  the 
south  side  of  them,  one  of  the  chief  indications  being  the  fact 
that  in  spite  of  a  strong  southerly  pitch  in  all  the  formations  they 
do  not  rapidly  disappear  below  sea  level. 

The  accompanying  profile  and  explanatory  section  indicates  the 
principal  results  of  exploration  [see  pi.  36].  Badly  crushed 
ground  has  been  found  in  the  holes  near  the  north  end  of  Morning- 
side.  Park  but  the  rock,  when  found,  is  not  very  badly  decayed. 
The  rock  floor  is  very  low,  almost  200  feet  below  sea  level  at  the 
lowest.  It  appears  that  if  the  drift  were  stripped  off  from  this 
valley  the  Hudson  and  Long  Island  sound  would  unite  across  the 
Harlem  Flats  and  Manhattanville  forming  a  channel  and  outlet 
much  deeper  than  the  present  East  river  course. 

The  glacial  drift  of  this  valley  is  prevailingly  fine  modified  drift 
some  of  which  is  probably  stratified  and  fairly  well  assorted. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


245 


This  is  more  strikingly  true  of  the  southerly  extension  of  this  low 
ground  southward  along  Morningside  Park.  A  very  deep  and 
prominent  preglacial  stream  came  down  from  the  gap  between 
Morningside  and  Central  Parks. 

It  is  not  yet  proven  that  the  fault  has  really  raised  the  Morning- 
side block.  At  least  if  there  is  such  displacement  it  is  not  of  suf- 
ficient amount  to  bring  up  a  different  formation  at  any  point- 
yet  examined.  It  would  be  possible  for  the  limestone  to  be  brought 
up  to  the  surface,  but  except  for  a  few  pieces  of  interbedded  lime- 
stone no  evidence  has  been  secured.  The  occurrence  of  this,  how- 
ever, is  thought  to  indicate  proximity  to  the  limestone  contact. 

General  geologic  conditions  established.  Fourteen  borings 
have  been  made  for  the  special  purpose  of  determining  exact  con- 
ditions. On  the  data  of  these  holes  there  are  several  features  now 
established  beyond  question  that  were  originally  given  only  as  prob- 
abilities. The  most  important  of  these  may  be  enumerated  as  fol- 
lows : 

1  A  very  deep  cross  valley  is  now  proven  between  123d  and 
126th  streets,  and  its  profile  can  be  plotted. 

2  A  part  of  this  ground  is  badly  broken,  as  if  belonging  to  a 
fault  zone,  but  most  of  the  floor  thus  far  tested  is  not  in  had  con- 
dition, i.  e.  it  is  not  very  badly  crushed  or  decayed. 

3  The  drift  cover  in  this  cross  valley  is  more  than  200  feet  deep 
over  a  distance  of  more  than  two  blocks  on  the  proposed  line  (from 
123d  street  to  Manhattan  street). 

4  The  limestone  contact  lies  more  than  300  feet  east  of  the  pro- 
posed line  at  this  Manhattanville  cross  valley. 

5  At  121st  street  the  limestone-schist  contact  stands  very  steep 
and  is  probably  slightly  overturned.  This  is  indicated  by  the  data 
of  hole  no.  33. 

6  The  contact  line  approaches  nearer  to  Morningside  Park  in 
passing  southward,  touching  the  park  between  110th  and  113th 
streets  and  the  contact  is  probably  not  overturned  in  this  southerly 
extension. 

3  Morningside  to  Central  Parks 

The  contact  between  In  wood  limestone  and  Manhattan  schist 
follows  nearly  parallel  with  the  Morningside  Park  boundary  on 
the  east  side,  but,  because  of  its  form,  actually  touches  the  park 
only  at  the  southern  end  between  noth  and  113th  streets.  At  the 
north  end  it  lies  off  more  than  half  a  block  to  the  east.  The  Man- 
hattan schist  forms  an  escarpment  because  of  its  more  resistant 


246 


NEW  YORK  STATE  MUSEUM 


character  and  this  eastward  facing  cliff  and  slope  forms  Morning- 
side  Park.  St  Nicholas  Park,  farther  north,  from  128th  to  155th 
streets  has  the  same  structural  relations.  In  both  cases  the  present 
escarpment  stands  back  from  200  to  500  feet  from  the  actual 
contact. 

As  the  formations  all  pitch  southward  and  are  pretty  closely- 
folded,  the  higher  formations  gradually  appear  and  at  110th  street 
another  parallel  ridge  of  Manhattan  comes  in  above  the  limestone 
in  the  trough  of  the  next  syncline  to  the  east.  This  forms  the 
north  end  of  Central  Park  and  from  this  point  southward  Man- 
hattan schist  is  continuous.  But  between  the  Morningside  belt  of 
schist  and  the  Central  Park  belt  at  110th  street  lies  an  anticline 
of  Inwood  limestone  also  pitching  southward  and  gradually  pass- 
ing beneath  the  schist  which  encroaches  upon  it  in  a  long  wedge 
until  a  few  blocks  farther  south  it  passes  wholly  beneath  the  schist, 
which  from  that  point  is  continuous. 

This  anticlinal  wedge  and  its  accompanying  structures  and  rock 
condition  was  the  subject  of  some  detailed  exploration. 

The  records  of  a  few  drill  holes  together  with  an  interpretation 
of  all  the  data  will  serve  for  the  present  purpose. 

The  most  important  borings  are  summarized  below : 

a  Hole  no.  3  on  113th  street,  232  feet  east  of  Morningside  Park 
East 

Surface  elevation-l-42.6  feet 
Rock  floor  at  depth  of  81.5  feet=el. -38.9  feet. 
Material : 

0-19  feet=to  eL+23.6  feet=soil  and  mixed  drift 
19-79  feet=to  el.  -36.9  feet=modified  drift.    Assorted  sands 
and  silts 

81.5-94.58  feet=to  el.  -54  feet  =  Inwood  limestone.  Typical 
and  in  good  condition 
b  Hole  no.  7.   On  113th  street,  corner  of  Manhattan  avenue 
Surface  elevation+38  feet 

Rock  floor  at  depth  of  approximately  165  feet=el.  -127  feet 
Material : 

0-85  feet=to  el.  -47  feet=modified  drift 

85-165  feet=to  el.  -127  feet=sand  with  much  more  clay,  part 

of  which  may  be  decayed  rock 
165-240  feet=to  el. -202  feet=disintegrated  rock  ledge.  Some 

micaceous  type  believed  to  be  the  transitional  facies  of  the 

schist-limestone  contact 


N.  Y.  State  Museum  Bulletin  146 


Plate  36 


TERRAGE 


t-J~i  L_J  ISIi  Lsjj  i_iL  t_Jio  1  ,  .AVE,  ^  ^  i_ 

"1  □  n  (Tj  m  ra  R]i^  r 
^gS^rm'ra  41  n  n  i i  r 

"T^Ti^  K  ^   #         i  g33  If- 


m 


uy     •■■     1  mi  t  if  1 1  [ 
-j   MOBMN6S10E  p*g£ 

n3!  I — 1  r~p]  1 — 1  f-pi  r" 


Buried 
Channel 


Geologic  detail  of  the  Manhattanville-Morningside  section  showing  the  alternative  lines  studied,  the  locations  of  exploratory  borings, 

principal  crusR  zones  and  longitudinal  profiles 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


247 


242-280.71  feet=-to  el.  -242.71  feet=Inwood,  very  coarse  type 
of  limestone.    Poor  core  showing.    Muoh  broken 
c  Hole  no.  12.    In  Morningside  Park  at  113th  street 
Surface  elevation+28  feet 
Rock  floor  at  depth  of  84  feet=el.  -50  feet 
Material : 

0-26  feet=to  el.+2  feet=mixed  drift 
26-84  feet=to  el.— 56  feet=modified  drift 

84-335.15  feet=to  el.-307.15  feet= Manhattan  schist,  typical 
with  considerable  pegmatite.    But  all  good  sound  rock,  not 
much  broken  and  standing  at  about  65°-8oG 
d  Hole  no.  16.    Corner  of  Manhattan  avenue  and  110th  street 
Surface  elevation=+55  feet 
Rock  floor  at  depth  of  159  feet— el-104  feet 
Material : 

0-44  feet=to  el.+il  feet=filled  ground  and  mixed  material 
44-159  feet=to  el-104  feet=fine  sands  and  silts  interpreted 
as  chiefly  modified  drift.  Much  of  it  very  fine  and  the  lower 
portion  rather  micaceous  and  angular  throwing  a  little  doubt 
on  the  exact  line  of  demarcation  between  drift  and  residuary 
matter 

159-161  feet=to  el-106  feet=core  of  Manhattan  schist 

1 71-186  feet=to  el.— 131  feet=decayed  rock  in  place,  some 
micaceous  type,  coming  out  as  mud 

186-228  feet=to  el.— 1 73  feet=micaceous  reddish  mud  with 
variable  amounts  of  angular  quartz  grains.  Certainly  residu- 
ary decayed  rock 

228-270  feet=to  el-215  feet=similar  residuary  matter  less 
highly  colored  passing  from  reds  into  grays  and  coming  out 
as  soft  material 

270-305  feet=to  el-250  feet=grayish  micaceous  and  quartz- 
ose  residuary  matters.  With  much  silvery  mica  and  chloritic 
grains  near  the  bottom 

305-335  feet=;to  el-280  feet=Inwood  limestone,  core,  ordi- 
nary type.    No  more  recovered  above  this  point  except  for  2 
feet  between  159  and  161  feet 
c  Hole  no.  36  at  108th  street  and  Manhattan  avenue 

Elevation  of  surface+63  feet 

Rock  floor  (decayed)  at  depth  of  55  feet— el .+8  feet 
Depth  to  solid  core=248  feet^el-185  feet 


248 


NEW  YORK  STATE  MUSEUM 


Material : 

0-55  feet=el.+8  feet=modified  drift  (fine  silts) 

55_155  feet=:to-io8  feet=micaceous  soft  material  with 
broken  sand=decayed  micaceous  rock 

1 55—2 1 5  feet=to-i52  feet=reddish  mud  of  similar  constitu- 
ents.   Is  decayed  rock  colored  by  iron 

215-240  feet=to-i  77  feet=transition  to  ir.ore  grayish  and 
greenish  soft  matter 

240-245  feet=to-i82  feet=greenish  mica  rock=a  decayed 
chlorite,  mica  quartz,  schist  layer 

248.33-254.25  feet=from  el.-185.35  to-191.25  feet=chloritic 

Inwood  limestone 

A  summary  of  these  data  gives : 
0-55  feet=drift 

55-245  feet = decayed  rock  ledge 
248-254  feet=solid  rock  ledge  (limestone) 
/   Hole  no.  2  at  123d  street,  100  feet  east  of  Morningside  Park  East 
Surface  elevation+30  feet 
Rock  floor  at  depth  of  220  feet=el.-i90  feet 
Material : 

0-13  feet=to  eL+17  feet=soil  and  mixed  drift 

13-220  feet=to  el-190  feet=modified  drift=mostly  assorted 

sands  and  silts 
220-245  feet=to  eL-215  feet=soft  decayed  schist 
245-355  feet=to  el-325  feet=Manhattan  schist  much  broken 

—  poor  core  recovery  —  worst  material  at  about  225-240  feet 

and  again  near  bottom.    Formation  evidently  much  shattered 

and  considerably  decayed 
g    Hole  no.  33  on  121st  street,  300  feet  east  of  Morningside  Park 

East 

Surface  elevation+31  feet 
Rock  floor  at  depth  of  195  feet=el.-i64  feet 
Material : 
0-25  feet — soil  and  mixed  drift 

25-195  feet=to  eL-164  feet— drift,  mostly  modified  drift= 

assorted  sands  and  fine  silts 
190-195  feet  coarser  material — pebbles 

195-200  feet=to  el-169  feet— Inwood  limestone,  coarser  lime- 
stone of  usual  type 

200-237  feet=to  el-206  feet=Manhattan  schist 

Ordinary  type  and  in  good  condition  [for  interpretation  see 
later  comments] 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


249 


Condition  of  the  limestone  schist  contact.  The  finding  of 
Inwood  limestone  above  the  Manhattan  schist  in  hole  no.  33  at 
121st  street  east  of  Morningside  and  the  fairly  sound  condition  of 
both  typeis  raises  the  general  question  of  the  condition  of  contact 
zones  as  compared  with  fault  zones. 

There  are  three  important  facts  to  consider  bearing  on  this  case : 
(1)  The  contact  zones  are  commonly  weaker  than  either  formation 
alone  and  (2)  at  this  particular  point  an  abnormal  relationship  is 
shown  by  the  overturned  strata  (the  limestone  lying  above),  and 
(3)  the  fault  zones  are  always  weak  and  extensively  decayed. 

Because  of  the  abnormal  position  of  the  limestone  here,  lying  as 
it  does  overturned,  a  weaker  more  pervious  rock  upon  a  more  sub- 
stantial and  less  pervious  one,  it  appears  to  be  reasonable  enough  to 
find  the  limestone  and  schist  fairly  well  preserved,  under  conditions 
where  a  vertical  or  a  normal  position  would  have  encouraged  decay 
because  permitting  a  more  ready  circulation. 

But  there  is  a  further  conclusion  that  seems  allowable,  i.  e.  the 
fault  or  crush  zones  are  more  extensively  decayed  than  the  simple 
contact  or  transition  zones.  And  contrariwise,  where  an  especially 
extensive  decay  is  encountered,  it  probably  is  to  be  associated  with 
a  crush  zone  due  to  fault  movement  rather  than  with  any  other 
structure. 

A  further  inference  seems  allowable  from  the  data  of  these  holes. 
It  is  probable  that  these  fault  zones  do  not  follow  the  contacts  or 
bedding  exactly  but  cut  across  at  low  angles,  sometimes  coinciding 
with  the  contact  lines  and  sometimes  falling  wholly  within  the  lime- 
stone or  the  schist. 

Great  depth  of  decay  at  south  end  of  Morningside  Park.  The 
finding  of  approximately  150  feet  of  decayed  rock  in  hole  no.  16 
and  of  nearly  200  feet  of  similar  type  in  hole  no.  36,  all  so  rotten 
that  the  material  came  up  as  a  mud,  raises  a  very  difficult  question 
as  to  the  conditions  that  make  such  extensive  decay  possible. 
Hole  no.  7  (113th  st.)  shows  extreme  decay  to  elevation -204  feet 
Hole  no.  16  (noth  st.)  shows  similar  condition  to  elevation -250 

feet 

Hole  no.  36  (108th  st.)  shows  similar  condition  to  elevation -185 
feet 

These  three  holes  showing  similar  condition  of  very  deep  decay 
are  located  almost  exactly  in  line.  Nothing  on  either  side  of  this 
line  is  in  so  poor  condition. 

Consideration  of  these  conditions  can  not  fail  to  raise  certain 
questions  of  interpretation. 


250 


NEW  YORK  STATE  MUSEUM 


1  It  would  appear  that  at  least  one  of  these  borings  (no.  7)  is 
near  the  schist-limestone  contact.  May  they  all  lie  then  in  the 
weakened  contact  zone? 

2  It  is  true  that  at  least  one  core  (also  from  no.  7)  shows  a 
badly  broken  condition.    May  they  all  lie  in  a  fault  zone? 

3  There  is  no  reasonable  doubt  but  that  the  geologic  structure  at 
the  south  end  of  Morningside  Park  is  that  of  a  pitching  anticline 
carrying  the  limestone  beneath  the  schist  in  its  southward  extension. 

May  the  excessive  decay  be  due  to  this  relation  ? 

The  evidence  on  these  various  possibilities  is  not  complete  enough 
to  make  a  conclusion  very  reliable.  But  there  are  two  or  three 
factors  that  have  a  bearing  and  they  unite  pretty  well  in  supporting 
one  view. 

These  factors  are:  (a)  the  exact  alinement  of  these  three  holes, 
(b)  the  crushed  core  of  hole  no.  7.  (c)  the  overturned  position  of 
the  formations  10  blocks  farther  north,  (hole  no.  33),  together 
with  the  apparently  normal  position  in  hole  no.  16. 

All  of  these  points  are  consistent  with  the  opinion  that  we  have  to 
do  here  with  the  crush  zone  of  a  fault,  one  that  runs  rather  straight 
and  one  that  follows  not  far  from  the  contact  of  the  schist  and  lime- 
stone at  this  point.  And  it  is  probable  that  the  weakness  follows 
the  west  margin  or  limb  of  the  limestone  anticline  as  it  plunges  be- 
neath the  schist.    Such  evidence  as  there  is  favors  this  view. 

If  that  is  true,  then  one  may  expect  that  the  worst  ground  is  not 
very  wide,  but  that  one  probably  can  not  go  entirely  around  it.  The 
best  line  would  run  south  far  enough  to  get  above  the  limestone, 
and  then  cut  across  the  weak  zone  nearly  at  right  angles.  It  is  cer- 
tain that  the  ground  improves  southward. 

Later  borings  are  all  confirmatory  of  the  conclusion  that  the  weak- 
ness is  narrow  and  dies  out  rapidly  southward  as  soon  as  the  lime- 
stone passes  well  beneath  the  schist.  No  bad  ground  has  yet  been 
found  on  106th  street  where  the  tunnel  will  probably  be  located. 

4  The  East  river  section 

Preliminary  studies  of  southern  Manhattan  and  the  East  river 
led  originally  to  the  conclusion  that  the  portion  of  the  East  river 
forming  the  great  eastward  bend  from  32c!  street  to  Brooklyn  bridge 
probably  has  a  simpler  geologic  structure  than  those  portions  farther 
north  or  south.  It  was  long  known  that  the  structure  at  Black- 
wells  Island  is  very  complex  and  involves  all  of  the  local  formations 
in  close  folding  and  considerable  faulting.    But  there  seemed  to  the 


3 

S 

<u 
3 


IX! 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


writer  after  studying  all  available  data,  good  reason  to  believe  that 
the  river  leaves  this  belt  when  it  bends  to  the  eastward  and  that  it 
i>  in  this  part  a  displaced  stream.  In  that  case  the  East  river  coidd 
bo  flowing  upon  a  floor  of  gneiss  of  a  most  substantial  sort. 

Explorations  are  now  complete  on  a  line  that  crosses  the  river 
from  Clinton  street,  Manhattan,  to  Bridge  street,  Brooklyn.  All 
borings  have  found  good  sound  rock  at  moderate  depth  and  all  are 
comparatively  shallow  holes.  Their  positions  and  depths  and  rock 
types  are  tabulated  below. 


No.  of 
bori.  g 

Dist  nces  in  feet 
fr-m  Manh-ttan 
pier  head  l.ne 

A  pproximate 
interval  in 
feet 

Elevation  of 

rock  floor 
below  mean 
sea  level  in 
Let 

Type  cf  rock 

Formation 

9 

0 

-48 

Granodiorite 

Fordham 

2  I 

225 

225 

-65 

S3 

35° 

125 

—  72 

ii 

32 

525 

175 

—  71 

5° 

695 

1  70 

-76 

u 

34 

860 

—  74 

tt 

41 

960 

100 

—  81 

39 

1  070 

1  10 

-67 

u 

67 

Brooklyn  side 

—  75 

Banded 

ii 

near  bulkhead 

gneiss 

The  rock  floor  is  thus  very  uniform  as  to  contour  across  the  East 
river  at  this  point.  No  water  course  yet  explored  about  Manhattan 
island  has  shown  so  simple  conditions  including  as  it  does  sound 
rock  and  shallow  channel.  The  rock  varies  a  good  deal  but  is  pre- 
vailingly a  coarse  grained  granodiorite.  In  places  it  is  very  gar- 
netiferous  and  at  others  is  banded  or  micaceous,  but  all  belong  to 
the  Fordham  formation  as  a  general  formational  unit. 

Borings  in  the  East  river  made  by  the  Public  Service  Commission 
both  above  and  below  this  point  found  an  occasional  deep  hole  with 
excessive  decay  to  more  than  a  hundred  feet  without  securing  sound 
core.  At  this  crossing  the  deepest  point  in  the  channel  to  sound 
rock  floor  is  81  feet. 

It  is  certain  from  these  results  and  from  others  in  adjacent 
ground  that  the  East  river  does  not  occupy  in  this  part  of  its  course 
the  original  stream  channel.  Tt  has  been  displaced  (evicted)  by 
glacial  encroachment  and  has  never  been  able  to  reoccupy  the  lost 
course.    Therefore,  instead  of  the  river  following  a  belt  of  lime- 


NEW  YORK  STATE  MUSEUM 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


253 


stone  around  this  big  bend  as  was  formerly  supposed,  it  follows  no 
rock  floor  structure  at  all  but  is  in  this  part  of  its  course  wholly 
superimposed.  The  original  valley  lies  farther  to  the  west  cutting 
through  the  midst  of  the  Lower  East  Side  where  the  more  com- 
plicated geologic  structures  again  prevail. 

Borings  at  intervals  of  500  feet  have  now  been  made  on  the 
Brooklyn  side  of  the  East  river  to  Gold  street  and  Myrtle  avenue. 
So  far  as  developed  there  is  no  other  formation  than  the  Fordham 
and  the  associated  granodiorite  within  the  area  covered.  The  rock 
floor  is  remarkably  uniform  at  an  elevation  of  from  -70  to  -90 
feet.  The  accompanying  section  shows  the  relations  of  rock  floor 
to  present  drift  surface  [fig.  40]. 

STRUCTURAL  GEOLOGY  OF  THE  LOWER  EAST  SIDE, 
DELANCEY  AND  CLINTON  STREET  SECTION 

The  proposed  distributary  conduit  turns  from  the  Bowery  east- 
ward on  Delancey  to  Allen  street,  thence  on  Allen  to  Hester  street, 
thence  on  Hester  to  Clinton  street  and  follows  south  on  Clinton 
to  the  East  river.  This  so  called  Lower  East  Side  section  includes 
one  of  the  most  complicated  geologic  structures  in  New  York  city. 
The  most  complex  portion  extends  from  Christie  street  on  the  west 
side  to  Monroe  street  on  the  east.  Between  these  two  points  all  of 
the  crystalline  rock  formations  form  a  series  of  parallel  beds  that 
are  folded  together  so  closely  that  they  stand  practically  on  edge. 

This  general  fact  and  the  approximate  location  of  the  several 
beds  have  been  proven  for  some  time.  But  the  more  exact  structure, 
with  the  depths  to  which  the  beds  go  before  bending  upward  again, 
and  the  distances  through  each  one  are  only  approximately  deter- 
mined by  the  exploratory  borings  to  date.  The  chief  uncertainties 
arise  from  the  fact  that  the  beds  are  also  faulted  and  the  dips  of 
the  fault  planes  are  not  yet  determined  and  the  amount  of  displace- 
ment is  unknown.  The  difficulty  of  forming  a  good  estimate  of  the 
obscure  points  is  greatly  increased  by  the  fact  that  no  rock  of  any 
kind  is  to  be  seen  at  the  surface.  Judgment  is  based  wholly  on 
borings. 

There  are  other  important  questions  covering  the  zone,  such  as : 
(1)  depth  of  serious  decay,  (2)  location  and  width  of  these  decay 
belts,  (3)  general  physical  condition  of  the  rock  at  certain  levels, 
(4)  length  of  tunnel  that  will  cut  each  formation,  (5)  best  depth 
for  safe  construction. 

The  accompanying  geologic  cross  section  [pi.  38]  embodies  an 
opinion  of  the  structural  relations  of  the  different  formations.   It  is 


254 


NEW  YORK  STATE  MUSEUM 


offered  as  the  writer's  interpretation  of  borings  to  date,  and  its 
more  direct  use  is  as  a  working  basis  and  guide  in  conducting  ex- 
plorations. The  western  half  of  the  section  may  be  accepted  as 
more  accurate  in  minor  detail  than  the  eastern. 

To  simplify  the  section  it  is  drawn  on  a  line  crossing  this  zone 
more  directly  than  the  conduit  as  laid  out,  i.  e.  through  holes  28  and 
5  and  the  borings  are  projected  along  the  strike  of  the  formation 
to  the  section  line.  All  the  data  therefore  are  used  and  the  structure 
is  not  distorted,  but  the  distances  through  each  bed  would  be  greater 
on  the  conduit  line  because  it  runs  more  diagonally  across  the 
formations. 

Borings.  The  following  tabulation  of  borings  and  interpre- 
tations upon  them  forms  the  basis  of  the  present  ideas  of  structure 
and  quality  of  rock  on  the  Lower  East  Side.  The  borings  are  given 
in  order  from  west  to  east,  and  all  points  are  neglected  except  those 
bearing  upon  geologic  structure. 
28  The  Bowery  and  Delancey  street 
Surface  elevation  40.5  feet 

Rock  floor  -71  feet.    Rock  is  Manhattan  schist,  and  has  been 
penetrated  to  -91  feet 
78  Delancey  street  west  of  Christie 
Surface  elevation  41.4  feet 

Rock  floor  -101.6  feet.    Rock  all  typical  Manhattan  schist  — 
at  about  6o° 

224  and  301  North  side  of  Delancey  street  west  of  Christie  street 

Surface  elevation  42  feet 

Rock  floor  at  el.  -99  feet 

Manhattan  schist  to  el— 330  feet 

Inwood  limestone  to  bottom  at  el.  -395  feet 
229  Northeast  corner  Delancey  street  and  Christie  street 

'Surface  elevation  43  feet 

Rock  floor  at  el.  -108  feet 

Manhattan  schist  with  very  poor  core  recovery  to  el.  -260  feet 
Inwood  limestone  to  bottom  at  el.  -360  feet 
84  Delancey  street  east  of  Christie 
Surface  elevation  41.8  feet 

Rock  floor  -135  feet.    All  badly  decayed  schistose  rock,  of 
same  type  —  no  effervescence  —  red  color  —  soft  as  cheese 
to  -204  feet 
227  is  a  reoccupation  of  this  same  hole  84 

Inwood  limestone  was  found  below  el.  -250  feet  to  the  bottom 
below  el.  -300  feet 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


255 


63  Delancey  street  west  of  Forsyth 
Surface  elevation  43.2  feet 

Rock  floor  —141  feet.    Inwood  limestone  at  80 0  —  very  low 
saving  of  core 
72  Delancey  street  121  feet  east  of  Forsyth 
Surface  elevation  42.6  feet 

Rock  floor  -122  feet.    Very  noncommittal  rock,  one  piece  very 
good  Fordham  and  the  rest  not  decidedly  any  special  type 

Classified  as  Fordham  on  this  basis.    Same  behavior  to  bottom 
-109  feet 
81  Delancey  street  and  Eldridge 

Surface  elevation  41.7  feet 

Rock  floor  -98  feet.   Rock  is  typical  Fordham  gneiss  —  banded 
and  very  micaceous  —  to  bottom  —123  feet 
225  North  side  of  Delancey  street  east  of  Eldridge  street 
Surface  elevation  40  feet 
Rock  floor  at  el.  -74  feet 

Fordham  gneiss  in  good  condition  with  interbedded  limestone 
at  bottom  at  el. -550  feet  to  bottom  at  el. -671  feet 
25  Delancey  street  between  Eldridge  and  Allen  streets 
Surface  elevation  40.6  feet 

Rock  floor  -68.3  feet.    Banded  Fordham  gneiss  —  sound  rock 
—  dip  about  45 0 
233  South  side  of  Broome  street  east  of  Allen  street 
Surface  elevation  42  feet 
Rock  floor  at  el.  -96  feet 

Fordham  gneiss  with  good  core  recovery  down  to  el.  -200  feet 
This  hole  is  also  known  as  302  under  a  subsequent  contract 
85  Delancey  street 

Surface  elevation  38.7  feet 

Rock  floor  -82.3  feet.    Banded  Fordham  gneiss  —  dip  about 
6o°  or  less  —  bottom  at  -171  feet 
223  Grand  street  east  of  Allen  street 
Surface  elevation  41.2  feet 
Rock  floor  at  el. -123  feet 
No  core  recovered  in  the  first  140  feet 

Inwood  limestone  with  dip  averaging  about  45°  from  el.  -303 

feet  to  bottom  at  el.  -710  feet 
Splendid  core  recovery 


256 


NEW  YORK  STATE  MUSEUM 


208  Hester  street  east  of  Allen  street 
Rock  floor  at  about  -145  feet 

Inwood  limestone  with  structure  at  about  60  feet  —  700 
Enters  fairly  sound  rock  and  has  continued  to  over  600  feet 
with  dip  as  low  as  200,  toward  the  bottom 
15  Delancey  street  near  Ludlow 
Surface  elevation  35.7  feet 

Rock  floor -106  feet.    Pegmatite  and  Inwood  limestone,  mas- 
sive and  bedding  obscure 
217  Southwest  corner  of  Ludlow  and  Hester  streets 
Surface  elevation  36  feet 
Rock  floor  at  el. -128  feet 

Inwood  limestone  for  more  than  a  hundred  feet  succeeded  by 
thin  strips  of  gneiss  and  limestone   interpreted   as  inter- 
bedded  Fordham 
222  Hester  street  west  of  Essex  street 

Surface  elevation  36.6  feet 

Rock  floor  at  el. -130.4  feet 

Fordham  gneiss  with  interbedded  limestone  showing  fair  core 

recovery  below  el.  -400  feet 
Dip  of  rock  structure  about  6o° 
216  South  side  of  Hester  street  between  Essex  and  Suffolk  streets 
Surface  elevation  33  feet 
Rock  floor  at  el. -167  feet 

Interbedded  limestone  and  Fordham  gneiss  with  a  dip  of  ap- 
proximately 450  to  el. -625  feet 
Core  recovery  was  variable 
8  Norfolk  and  Grand  streets 
Surface  elevation  35.8  feet 

Rock  floor  -130  feet.   Rock  a  close  grained  schistose  limestone, 
Inwood,  showing  foliation  at  about  45 0 
231  South  side  of  Hester  street  opposite  Norfolk  street 
Surface  elevation  32  feet 
Rock  floor  at  el.  -103  feet 

Decayed  gneiss  and  no  core  recovery  to  el.  -300  feet.  This 
boring  was  continued  as  no.  303  under  a  subsequent  con- 
tract and  carried  to  el.  -525  feet  with  only  a  small  recovery 
of  Fordham  gneiss 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


257 


218  South  side  of  Hester  street  east  of  Norfolk  street 
Surface  elevation  31  feet 

Rock  floor  at  el.  -183  feet 

No  core  recovered  in  upper  300  feet 

Interbedded  limestone  in  Fordham  gneiss  below  el.  -550  feet 
to  bottom 

77  Hester  street  near  Suffolk  [see  202] 
202  Hester  street  west  of  Suffolk 
Surface  elevation  30.5  feet 

Rock  floor  -99.5  feet.    Rock  all  decayed  to  great  depth 
Manhattan  schist  to  -470  feet 
Fordham  gneiss  —470  feet  to  bottom  at  -577  feet 
Believed  to  cross  fault  plane 
213  Hester  street  85  feet  east  of  Suffolk  street 
Surface  elevation  33.3  feet 
Rock  floor  at  el. -116. 7  feet 

The  rock  is  Fordham  gneiss  of  the  black  and  white  banded 
type,  with  dips  varying  from  300  to  8o°.  For  a  very  short 
distance  at  el.  -275  feet  dips  of  io°-i5°  were  recorded 
Core  recovery  very  good 

11  Hester  and  Clinton  streets 

Rock  floor  -204  feet.    Badly  disintegrated  and  no  core  to  -279 
feet.     Unusual  rock,  identified  as  a  mica  schist  of  obscure 
structure  (not  typical).    Some  calcareous  portions. 
At  first  this  was  thought  to  belong  to  the  Manhattan  formation, 
but  it  is  probably  a  schistose  and  rather  unusual  facies  of  the  Ford- 
ham series.    This  hole  was  subsequently  reoccupied  and  deepened 
as  no.  220  under  another  contract  with  the  result  that  an  inter- 
bedded series  of  gneisses  and  limestones  was  shown  to  a  total  final 
depth  reaching  el  -660  feet.    Rock  cores  indicate  dip  of  about  6o°. 
201  Clinton  street  between  east  Broadway  and  Henry  street 
Surface  elevation  —31.3  feet 
Rock  floor  -133.7  feet 

A  schistose  variety  of  Fordham  gneiss  with  associated  inter- 
bedded limestone 

219  Northwest  corner  of  Clinton  and  Madison  streets 
Surface  elevation  26  feet 

Rock  floor  at  el.  -214  feet 

Fordham  gneiss  and  interbedded  limestone 

Good  core  recovery  below  el.  -400  feet 


258 


NEW  YORK  STATE  MUSEUM 


232  Southeast  corner  of  Clinton  and  Madison  streets 
Surface  elevation  25  feet 

This  hole  was  reoccupied  as  no.  304  and  penetrated  the  rock 
floor  at  el.  -353  feet 

The  boring  has  not  progressed  far  enough  to  recover  identifi- 
able material  for  rock  formation 

211  East  side  of  Clinton  street  south  of  Madison 
Surface  elevation  24 

Rock  floor  elevation  uncertain  because  of  failure  to  recover 
core  and  the  obscurity  of  the  material  washed  up.  Inter- 
bedded  limestones  and  gneisses  of  Fordham  series  were 
recognized  from  el.  -336  feet  to  el.  -680  feet 
51  and  207  Henry  street  between  Clinton  and  Montgomery 

Surface  elevation  32.4  feet 

Rock  floor  -214.6  feet.  All  badly  decayed  to  great  depth  mostly 
believed  to  belong  to  limestone  and  underlain  by  interbedded 
Fordham  gneiss  at  about  -345  feet 
221  Clinton  street  near  Monroe  street 
Surface  elevation  22  feet 
Rock  floor  at  el.  -116  feet 

Fordham  gneiss  mostly  very  sound,  with  some  thin  interbeds 

of  limestone  at  about  el.  -500  feet 
Dip  of  structure  450  to  8o° 
226  West  side  of  Clinton  street,  north  of  South  street 
Surface  elevation  10  feet 
Rock  floor  at  el.  -37  feet 

Fordham  gneiss  in  very  sound  condition  showing  structure  at 
6o°  to  90 0 

305  Southwest  corner  of  Clinton  and  South  streets 
Surface  elevation  9  feet 
Rock  floor  at  el.  -50  feet 
Fordham  gneiss  with  structure  at  700 

4  Montgomery  and  Madison  streets 
Surface  elevation  -32.5  feet 

Rock  floor  -65.5  feet.    Fordham  gneiss  of  granodiorite  type 
Two  borings  are  of  special  interest  and  significance,  and  because 
of  the  rarity  of  such  details  being  recorded  they  are  given  more 
fully  below. 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


Each  one  is  of  great  depth  and  indicates  conditions  decidedly 
different  from  the  commonly  accepted  behavior  for  Manhattan. 
Inland. 

Special  interpretation  of  hole  no.  202,  on  Hester  st.  near 
Suffolk  st.  This  is  one  of  the  very  deep  borings,  on  the  pro- 
posed distributary  conduit,  put  down  to  investigate  the  character, 
condition,  and  structure  of  the  rock  through  which  the  proposed 
tunnel  would  pass. 

A  summary  of  the  data  secured,  together  with  an  interpretations 
of  conditions  encountered  follows : 
I  Boring  record  (summary) 

Elevation  of  surface  =  +30.5 
a  Glacial  drift 

0—123  feet.    Soil  and  various  types  of  glacial  drift 
b  Residuary  matter  of  local  decay 

130-150  red  micaceous  mud 
c  Disintegrating  rock  ledge  too  much  decayed  to  furnish  core 
150— 190  disintegration  matter  from  pegmatite  and  associated* 
ledge 

190-214  quartz,  hornblende,  chlorite,  mica,  disintegration  sand5 
d  Decayed  ledge  rock  capable  of  furnishing  an  occasional  core 
214-224  core  —  several  pieces  of  coarse  feldspathic,  quartzv 
mica  rock 

224-237  core  —  several  pieces  of  core  with  much  green  mica- 
237-255  Cuttings  and  disintegration  sands  with  much  gree.* 
mica 

255-277  Pegmatite  cuttings 

277-305  Yellow  clays  and  quartzose  disintegration  sands  and 

cuttings 
305-314  Core-pegmatite 

314-388  Gray  quartzose  disintegration  sands 

402-447  Coarse  quartz  and  mica  disintegration  sands  and  finer 
quartz-mica,  hornblende-chlorite  cuttings  that  do  not  look 
badly  decayed.  The  rather  surprising  thing  is  their  failure 
to  core 

447-463  Core  —  four  pieces  of  schistose  rock  with  white  mica- 
and  garnet,  nearly  vertical,  and  three  fragments  of  pegmatite 
463-497  Cuttings  only 

9 


260 


NEW  YORK  STATE  MUSEUM 


497-5 12  Core  — a  quartz  biotite,  feldspar  schistose  rock  that  is 
rather  easily  disintegrated  but  does  not  show  bad 
decay.  Resembles  the  Fordham  formation  more 
than  the  Manhattan 

512-531  Disintegration  sand  and  cuttings  containing  abundant 
pearly  mica 

53 '-547  Core.    Many  fragments  of  coarse  quartzose  and  mica- 
ceous limestone,  interbedded  type 
e  Ledge  furnishing  sound  core 
558-559  Core  from  quartz  vein 

573-588  Close  textured  quartz  —  feldspar  —  mica  rock.  Two 
pieces  with  foliation  structure  at  about  6o° 

597-607  Typical  banded  Fordham  gneiss  with  good  structure. 

dip  about  6o°,  common  black  and  white  or  gray  and 
white  bands  in  good  solid  condition.  Thin  sections 
and  microscopic  examination  of  the  rock  indicate 
bottom  perfectly  crystalline,  well  interlocked,  fo- 
liated rock  with  constituents  in  good  sound  condition 

Summary  of  record  and  formation  assignment 

Feet 

0-123  Soil  and  drift 
130-150  Residuary  matter  of  local  decay 

150-500  Ledge  rock  considerably  decayed  —  micaceous  schist  pass- 
ing into  quartzose  schist  or  gneiss  mostly  badly  decayed, 
but  occasionally  giving  core 

500-531  Quartzose  rock  resembling  the  Fordham  rather  than  the 
Manhattan 

531-547  A  quartzose  limestone  probably  interbedded  with  the  Ford- 
ham 

558-607  Fordham  gneiss,  the  lowermost  part  of  which  is  very  sound 

Discussion  of  meaning  of  this  hole 
There  were  three  rather  puzzling  features  about  the  data  of  this 
hole  at  the  time  it  was  made:  (1)  The  fact  that  Fordha m  gneiss 
was  penetrated  at  a  point  so  far  to  the  west;  (2)  the  finding  of  a 
small  bed  of  quartzose  limestone  in  the  midst  of  other  types  ;  (3)  the 
finding  of  both  schistose  rock  closely  resembling  the  Manhattan  and 
typical  Fordham  gneiss  in  the  same  hole  with  so  little  space 
between. 

As  to  these  points,  the  first  one  needs  little  comment.  That  is,  it 
seems  to  mean  that  much  more  of  this  Lower  East  Side  ground  be- 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


tween  Madison  on  the  east  and  the  Bowery  on  the  west  belongs  to 
the  Fordham  than  at  first  supposed.  This  very  much  improves  the 
outlook  for  safe  and  easy  construction. 

The  second  one,  i.  e.  the  finding  of  limestone  at  531  feet  is  prob- 
ably an  interbedded  limestone  bed  and  not  a  part  of  the  large  In- 
wood  formation. 

The  third  point,  i.  e.  the  finding  of  schists  and  gneisses  in 
the  same  hole  introduced  more  difficulty  of  interpretation.  This  dif- 
ficulty was  considerably  increased  by  the  fact  that  the  ledge  is  so 
badly  decayed  and  so  broken  up  in  the  drilling  that  no  typical  ma- 
terial for  identification  could  be  secured  in  the  upper  portion.  There 
is  no  doubt  as  to  the  finding  of  Fordham  in  the  lower  portion. 
Later  explorations  support  the  conclusion  that  the  whole  belongs  to 
the  Fordham  series. 

When  this  boring  was  first  made,  the  schistose  portion  was. 
thought  to  be  the  Manhattan  formation,  and  the  limestone  could 
then  be  Inwood.  Subsequent  exploratory  work  at  other  points  has 
proven  that  the  Fordham  itself  shows  such  schistose  facies  rather 
commonly  where  associated  with  the  interbedded  limestones.  This 
is  now  the  accepted  interpretation  for  the  whole  eastern  half  of  the 
Lower  East  Side  belt  covered  in  the  present  discussion. 

There  probably  is  some  faulting.  But  whether  the  fault 
plane  dips  east  or  west  and  how  much  the  total  movement  is. 
has  not  yet  been  developed.  This,  however,  is  a  more  vital 
question  than  would  at  first  appear,  for  if  the  fault  dips  east 
the  ground  to  the  west  of  it  is  probably  all  Fordham  of  good 
quality  and  will  be  easily  explored,  whereas  if  the  fault  plane  dips 
to  the  west  the  whole  west  side  for  several  blocks  is  much  more 
uncertain.  It  is  probable  that  the  majority  of  the  rock  lying  west 
of  it  will  be  of  better  quality  than  found  in  this  hole. 

Interpretation  of  hole  no.  207  (old  no.  51) 
On  Henry  street  midway  between  Clinton  and  Montgomery 

I  )rill  boring  no.  207  has  been  put  down  to  a  depth  of  more  than 
655  feet  (approximately  -633).  The  material  is  of  unusual  quality 
and  behavior  and  therefore  seems  to  require  special  study  with  a 
view  to  reaching  a  correct  interpretation.  The  most  essential  points 
of  the  drill  record  are  given  below. 


262 


NEW   YORK  STATE  MUSEUM 


s.  Explanatory  record. 
m  Soil  and  glacial  drift  (surface  to  depth  of  195  feet) 

Surface  to  190  feet=sands,  gravels,  clays  of  unusual  variety 
190-195  feet=reddish  clay 
tb  Residuary  matter- — mostly  decayed  rock  (195-247  feet) 

-212-240  feet  micaceous  clay  —  judged  to  be  residuary  because 
of  the  abundance  of  mica  and  the  scarcity  of  worn  quartz 
grains  and  rarity  of  foreign  particles 
■c  Decayed  rock  ledge  preserving  original  structure 

representing  interbedded  limestone  (247-377  feet) 
247  feet=decayed  rock  ledge  with  white  blotches  showing 

traces  of  structure 
256-330  feet=oxidized — mostly  red  and  brown  clays  and  sands 

from  disintegration  of  decayed  rock  in  place 
349-351  feet=gray  micaceous  clay 

251-377  feet=quartzose  and  micaceous  disintegration  sands 
and  calcareous  clays  that  effervesce  in  acid.   Much  pearly  mica 
d  Decayed  rock  leclge  representing  Fordham  gneiss  formation 
(377-489  feet) — no  calcareous  matter 

377-489  feet=quartz  and  pearly  mica  disintegration  sand  vary- 
ing from  coarse  to  fine  and  mostly  of  very  light  buff  color 
r  Disintegration  matter  from  a  chloritized  hornblendic  gneiss  of 
too  little  cohesion  to  witb stand  the  grinding  action  of  a  drill 
of  so  small  cross  section  (13/16  inch).    (487-532  feet) 

487-532  fcet=fine  dark  colored  disintegration  sand  composed 
chiefly  of  quartz,  chlorite  and  mica,  the  material  is  of  same 
composition  as  the  cores  secured  just  below 
f  Core  from  more  substantial  rock  —  a  hornblendic  gneiss  sound 
enough  in  part  to  withstand  the  drilling  process  and  save  a 
small  amount  of  core  (532-655  feet) 

532—537  feet  —  9  pieces  of  a  green  chloritic  foliated  rock  (14 
inches-*-)  structure  7o°-8o° — a  close  textured  rock  much 
oxidized  and  hydrated 

537-551  feet  —  8  small  pieces  and  other  fragments  of  same 
rock 

551-566  feet — 17  pieces  and  several  fragments  sarqe  rock. 
All  close  texture  and  highly  chloritic 

581-596  feet  —  2  small  pieces  (two  very  brown,  hard  pieces) 
are  probably  not  natural  —  "  drillite,"  i.  e.  a  peculiar  product 
formed  by  the  drill  when  it  is  run  too  dry  and  partly  fuses 
fragments  of  rock  and  flakes  of  iron  from  the  drill  into  a 
-compact  rocklike  mass 


GEOLOGY  OF  THE  NEW 


YORK  CITY  AQUEDUCT 


263 


611-631  feet — 14  pieces  same  chloritic  foliated  rock.  Two 
pieces  of  "  drillite  " 

One  piece  of  fresh  rock  —  a  gray  gneiss  of  rather  worn  texture 

646-655.5  feet — 16  pieces  of  —  a  white  and  black  and  red 
blotched  rock  —  a  garnetiferous  gneiss.  The  rock  is  not  a 
common  type  but  a  similar  variety  is  sometimes  seen  along 
the  margins  of  the  granodiorite  intrusions  and  belongs  to  the 
Fordham  gneiss  series. 

Rock  is  fairly  sound  and  for  the  size  of  core  the  saving  is  good. 
(3  feet) 

2  Deflection  test.  A  deflection  test  on  this  hole  indicates  that  the 
drill  lias  not  departed  more  than  50  from  the  vertical. 

3  Behavior  of  drill.  It  has  been  possible  to  drive  the  casing  down 
after  the  drill  without  reducing  the  size  and  without  enlarging  the 
rock  hole  to  a  final  depth  of  625  feet. 

About  half  of  the  water  fed  into  the  machine  is  lost —  10  gallons 
per  minute  being  fed  and  $l/2  gallons  recovered. 

The  hole  filled  after  each  pull  up  as  much  as  100  feet  with  mat- 
ter that  either  ran  in  from  a  crevice  or  was  furnished  by  disinte- 
gration of  the  walls  or  was  simply  the  settling  of  matters  held  in 
suspension  during  operation.  These  settlings  or  corings,  as  the 
case  may  be,  were  of  large  amount  (100  feet  +  )  when  the  drill  was 
cutting  far  below  the  casing  and  small  in  amount  (5  feet)  when 
the  casing  was  driven  down  near  to  the  bottom.  This  matter  then 
increases  as  the  hole  is  deepened  again  below  the  casing. 

Cutting  and  progress  are  rapid  and  easy. 

4  Examination  of  the  rock,  (a)  Hornblendic  gneiss.  A  miero- 
scopic  examination  of  the  green  hornblendic  gneiss  shows  that  the 
rock  is  not  badly  crushed  and  that  the  different  original  grains  are 
well  interlocked.  But  the  more  easily  affected  mineral  constituents 
are  very  generally  decayed  and  have  become  especially  modified  on 
their  surfaces  where  they  interlock  with  other  grains.  The  matter 
developed  is  mostly  chlorite  ^-  a  mineral  that  is  very  soft  and  one 
that  in  this  case  fails  to  furnish  a  very  firm  bond  between  the  grains. 
A  disrupting  force  exceeding  the  strength  of  this  soft  mineral  there- 
fore, such  as  drilling  with  a  small  bit  or  forcing  the  drill,  causes  the 
grains  one  by  one  to  roll  out  or  break  apart  and  furnish  the  sus- 
pended matter  that  seems  to  be  so  abundant  in  this  hole. 

b  The  rock  below  646  feet.  This  is  a  very  unusual  type  of  rock, 
the  petrographic  character  of  which  need  not  be  taken  up  here.  It 
appears  to  be  simply  a  contact  variety,  such  as  sometimes  is  devel- 


264 


NEW  YORK  STATE  MUSEUM 


oped  along  the  margins  of  the  granodiorite  masses  where  they  cut 
into  the  banded  Fordham  gneiss. 

The  essential  feature  of  the  rock  is  its  fresh  and  sound  char- 
acter.   This  rock  is  not  decayed. 
5  Interpretation 

a  Drift 

The  glacial  drift  and  soil  cover  the  bed  rock  at  this  point  for  a 
depth  of  at  least  195  feet. 
b  Residuary  soil 

Decayed  residuary  matter  of  local  derivation  is  detected  at  212 
feet. 
c  Bed  rock 

The  decayed  matter  still  preserves  the  bed  rock  structures  in  a 
sample  taken  at  347  feet.    From  this  point  downward  there 
is  decayed  rock  ledge  gradually  becoming  more  substantial 
d  Formations  represented 

After  bed  rock  is  reached  the  first  100  feet  is  so  altered  that 
identification  is  not  certain.  At  350  feet,  however,  the  cal- 
careous nature  of  some  of  the  material  is  observed,  and  on 
this  ground  largely  it  is  believed  that  an  interbedded  lime- 
stone layer  is  penetrated  down  to  about  377  feet. 

From  that  point  (377  feet)  the  material  is  very  silicious  and 
not  at  all  calcareous  and  the  core  when  obtained  is  distinctly 
gneissoid.  This  lower  portion  below  (377  feet)  is  therefore 
judged  to  be  typical  Fordham  gneiss. 

The  bottom  material  is  sound  but  a  very  rare  variety  for  this 
formation. 
e  Character  of  contact 

Normally  the  interbedded  limestone  lies  conformable  to  the 
structures  and  beds  of  Fordham  gneiss.  The  structure  in 
such  pieces  as  show  it  indicated  a  dip  of  about  70-800. 
Therefore  the  formation  must  stand  very  steep.  But,  so  far 
as  can  be  seen  in  the  fragments  secured,  there  is  no  direct 
evidence  of  a  fault  contact  or  anything  abnormal.  The  ex- 
tremely deep  alteration  of  the  rock  is  the  chief  unusual  fea- 
ture. It  seems  to  require  a  better  chance  for  water  circula- 
tion than  is  natural  in  the  undisturbed  rock  of  either  forma- 
tion. For  this  reason,  I  am  of  the  opinion  that  there  has 
been  movement  in  this  zone  that  weakened  the  rock  enough 
to  encourage  water  circulation. 

The  formation  dips  west  in  normal  manner  at  about  75  degrees. 


GEOLOGY  OF  THE  NEW   YORK  CITY  AQUEDUCT 


265 


/  Condition  of  the  rock 

That  the  upper  100  feet  of  ledge  is  very  rotten  can  not  be  de- 
nied, but  it  is  certain  that  this  lower  portion  of  the  hole 
is  not  in  so  bad  condition  as  the  low  saving  of  core 
would  lead  one  to  think.  The  grains  are  affected  by  chloritic 
alteration  in  such  manner  that  they  can  not  resist  much 
disrupting  force.  The  small  diameter  of  drill  used  subjects 
the  whole  core  to  enough  strain  to  cause  the  gradual  pul- 
verization of  the  rock.  This  affects  both  the  core  that  has 
been  cut  loose  and  the  hole  wall  that  is  further  subjected  to 
the  thrashing  of  the  drill  rods.  A  larger  size  core  would 
make  a  very  much  more  encouraging  and  fair  showing. 

There  may  be  an  occasional  small  seam  so  badly  decayed  that 
it  is  encouraged  to  run  or  cave  under  such  treatment.  But 
there  is  absolutely  no  evidence  that  slumping  or  caving  is 
common  or  even  likely  on  any  considerable  scale. 

The  material  that  partly  fills  up  the  hole  when  the  drill  i 
pulled  up  is  believed  to  be  in  considerable  part  the  settlings 
of  suspended  matter  which  during  the  agitation  of  drilling  is 
distributed  through  the  rising  column  of  water.  The  reduc- 
tion in  volume  (10  gallons  being  fed  and  only  5V2  gallons 
being  recovered)  due  to  rock  porosity  is  favorable  to  such 
behavior  of  the  loosened  material. 

SUMMARY  OF  LOCAL  GEOLOGY. 

Formations.  Only  three  formations  are  represented  in  the 
rock  floor  of  this  section.  These  are  the  regular  crystallines  char- 
acteristic of  all  southeastern  New  York. 

1  Manhattan  schist 

2  Inwood  limestone  or  dolomite,  and 

3  Fordham  gneiss,  including  the  Ravenswood  granodiorite  as  a 
special  intrusive  member,  and  an  unusually  strong  development  of 
the  interbedded  limestones  and  associated  schistose  facies. 

These  formations  have  their  usual  relation  —  the  Manhattan 
above  and  youngest,  the  Inwood  intermediate,  and  the  Fordham 
underneath  and  older.  These  simple  relations,  however,  are  much 
complicated  by  dynamic  disturbances  of  more  than  usual  violence 
so  that  the  series  is  thrown  into  folds  so  close  that  the  individual 
beds  stand  almost  on  edge.  In  addition  lateral  thrusts  of  that  same 
time  or  later  have  broken  the  strata  and  faulted  them  in  several 
places.    This  complicates  the  structures  still  more,  and,  since  the 


266 


NEW  YORK  STATE  MUSEUM 


amount  of  displacement  is  in  no  case  fully  known,  makes  the  struc- 
tures in  some  minor  details  impossible  to  accurately  interpret  at  this 
stage  of  the  work. 

Fault  zones.  As  nearly  as  the  material  recovered  can  be 
classified  and  accredited  to  the  above  three  formations  it  has  been 
done.  On  this  identification  together  with  the  location  of  points  of 
greater  decay  the  chief  fault  zones  are  drawn.  The  chief  ones  are 
judged  to  be  thrust  faults  but  it  is  possible  that  one  is  a  normal 
fault.  Such  a  combination  is  comparatively  rare  where  the  zones 
are  so  close  together,  but  it  seems  to  best  explain  the  relations  of 
beds  as  interpreted  from  identification  of  the  present  borings.  It  is 
not  an  unknown  association  though  in  this  region.  It  probably  in- 
dicates faulting  in  two  different  periods.  This  is  consistent  with 
the  observation  also  that  some  of  the  fault  breccia  ground  is  not 
much  decayed  while  others  are  badly  affected.  Probably  the  later 
movements  have  not  allowed  rehealing  of  the  crevices  and  they  are 
then  the  lines  of  chief  circulation  and  alteration. 

It  is  clear,  upon  examination  of  the  section  as  now  known,1  that 
both  the  eastern  and  western  belts  of  limestone  are  too  thin  and 
narrow  to  accommodate  the  whole  Inwood  limestone.  The  Inwood 
normally  is  a  formation  of  about  750  feet  or  more  in  thickness.  It 
is  therefore  certain  that  a  part  of  it  has  been  cut  out  by  squeezing 
or  faulting.  If  by  faulting  then  there  would  be  expected  to  be  in 
each  case  somewhat  greater  decay  than  usual  along  the  fault  zones. 
The  fact  therefore  that  such  decay  zones  are  found  along  one  mar- 
gin of  the  limestone  bed  in  each  case  leads  to  the  conclusion  that 
faulting  is  the  true  cause.  In  some  cases  thrust  faulting  would  be 
required  to  produce  the  result  and  leave  the  beds  standing  in  their 
present  relations  [see  pi.  38]. 

INTERBEDDED  LIMESTONES  OLDER  THAN  THE  INWOOD 

The  finding  of  limestone  beds  within  the  Fordham  gneiss  forma- 
tion so  persistently  in  the  Lower  East  Side  borings  is  one  of  the 
geologically  interesting  and  rather  surprising  results  of  recent  ex- 
ploration. All  of  the  borings  in  the  Fordham  gneiss  area  in  this 
particular  district  except  those  near  the  East  river  have  shown  some 
limestone. 

The  individual  beds  vary  greatly  in  thickness,  ranging  from  only 
a  few  inches  to  many  feet.  Because  of  the  steepness  of  the  dip  of 
the  beds  and  the  obscurity  of  this  factor  in  many  borings  it  is  sel- 


October  1910. 


GEOLOGY  OK  THE  NEW  YORK  CITY  AQUEDUCT 


267 


dom  possible  to  compute  their  thickness  closely.  It  is  probable  that 
most  of  them  are  not  over  5  to  10  feet  thick,  although  rarely  a 
thickness  of  25  or  30  feet  may  be  represented.  It  is  certain  also 
that  a  considerable  number  of  separate  beds  are  penetrated.  Alt 
attempts  to  correlate  the  limestone  cores  from  different  adjacent 
holes  have  so  far  met  with  little  success.  No  doubt  some  of  those 
cut  at  great  depth  in  one  hole  correspond  to  others  cut  higher  ire 
an  adjacent  hole.  But  the  differences  in  thickness  are  notable  evert 
in  the  best  cases,  and  it  is  evident  that  little  dependence  can  be  put 
upon  uniformity  of  thickness  as  a  factor  in  correlation.  The 
foldings  and  crumplings.  and  shearing  have  probably  affected  the 
limestone  members  of  the  series  more  than  any  others.  Limestones 
in  comparatively  thin  beds  are,  under  such  conditions,  especially 
liable  to  excessive  thinning  and  thickening"  through  recrystalliza- 
tion  and  rock  flowage.  It  is  not  at  all  likely  that  any  single  bed  at 
present  preserves  much  uniformity  of  thickness.  In  some  places 
they  are  pinched  out  entirely  while  in  others  they  may  attain  a 
thickness  much  greater  than  the  original.  It  is  possible  also  that 
some  of  them  are  repeated  by  folding.  Whether  or  not  this  is  true 
in  the  Lower  East  Side  section  no  one  can  tell.  On  the  whole  there 
is  no  direct  evidence  of  repetition  in  this  way.  After  making  al- 
lowance for  all  possible  duplication  there  is  still  a  surprisingly  large 
number  of  limestone  interbeds  represented- — probably  10  —  a 
larger  number  in  succession  than  is  known  anywhere  else  in  south- 
eastern New  York  [see  pi.  38]. 

In  petrographic  character  these  so  called  limestones  are  all  es- 
sentially very  coarsely  crystalline  dolomitic  marbles  or  silicated  dolo- 
mites of  still  more  complex  constitution.  Occasionallv  a  very  pure 
carbonate  rock  is  represented  that  corresponds  in  appearance  very 
closely  indeed  to  the  best  grades  of  the  Inwood,  but  there  is  no  doubt 
whatever  of  the  true  interbedded  relation  of  these  limestones.  Their 
similarity  of  appearance  to  the  Inwood  in  certain  facies  is  so  great 
that  from  the  petrographic  evidence  alone  one  could  not  differen- 
tiate them.  Their  fixed  relation  however  is  unmistakable  and  they 
belong  unquestionably  to  an  entirely  different  geologic  formation^ 
from  the  Inwood  —  a  much  older  one,  in  fact  the  oldest  known 
formation  in  southeastern  New  York  —  equivalent  to  the  Grenville 
series  of  the  Adirondacks  and  Canada.  The  silicated  facies  con- 
tains many  of  the  common  products  of  metamorphic  processes- 
Recrystallization  has  produced  micaceous  minerals  such  as  phlogo- 
pite  and  chlorite  in  abundance.    Original  and  secondary  quartz  is 


^68 


NEW   VORK  STATE  MUSEUM 


plentiful.  Serpentine,  tremolite,  diopside,  actinolite,  occasionally 
chondrodite,  and  rarely  metalic  ores  are  found,  in  many  cases 
the  limestone  passes  by  transition  gradually  into  a  more  and  more 
silicious  faciei  until  the  rock  is  simply  a  silicious  Fordham  gneiss 
with  quartz,  mica  and  feldspar  as  the  essential  constituents.  There 
is  seldom  a  sharp  break  between  the  two  types.  Many  pieces  of 
apparently  simple  gneiss  will  show  effervescence  of  a  carbonate 
constituent  with  acid. 

The  silicious  beds  of  the  gneiss  series  proper  immediately  associ- 
ated with  the  limestone  layers  are  also  more  silicious  or  more  mi- 
caceous than  the  average  Fordham.  They  are  essentially  micaceous 
quartzites  and  mica  schists  and  the  rock  generally  lacks  the  strong 
black  and  white  banding  that  characterizes  the  common  or  typical 
Fordham  gneiss  of  other  localities.  It  is  this  facies  of  the  gneiss 
which  most  closely  resembles  certain  facies  of  the  Manhattan  schist, 
and  when  the  rock  is  much  decayed  or  badly  broken  or  is  ground 
to  pieces  by  the  drill  the  confusion  is  still  greater.  The  micaceous 
variety  may  readily  be  mistaken  for  Manhattan  schist  and  the  ac- 
companying limestone  may  equally  be  mistaken  for  Inwood. 

The  occurrence  of  interbedded  limestones  of  the  Fordham  series 
is  probably  more  common  than  was  formerly  believed.  They  are 
not  very  often  seen  on  the  surface  areas  of  gneiss.  Possibly  this 
i^  largely  due  to  differential  weathering  and  erosion  which  together 
tend  to  obscure  those  portions  of  outcrops  where  such  beds  maj 
occur.  But  the  type  is  well  known.  Mr  W.  W.  Mather  in  his 
Geology  of  the  First  Geological  District  [1843]  interpreted  certain 
limestones  in  the  Highlands  as  interbedded  in  their  relation  to  the 
gneisses  there.  Later  workers  were  inclined  to  disregard  his  views 
on  this  point  and  there  was  a  marked  tendency  to  place  all  lime- 
stone occurrences  in  one  formation.  Some  of  the  geological  maps 
have  been  made  in  this  way.  The  writer,  however,  raised  the  issue 
again  in  an  article  published  in  1907  under  the  title  "  Structural  and 
Stratigraphic  Features  of  the  Basal  Gneisses  of  The  Highlands,"  a 
N.  Y.  State  Museum  Bulletin  107.  It  is  certain  that  there  are  inter- 
bedded limestones  with  the  gneisses  in  The  Highlands.  More  re- 
cently, the  writer  has  recognized  similar  occurrences  in  the  typical 
Fordham  gneisses  of  The  Bronx,  New  York  city.  The  vicinity 
of  Jerome  Park  reservoir  is  the  best  locality  in  all  southeastern  New 
York  to  see  this  interbedded  development.  The  best  exposures  are 
at  the  following  places. 

1  In  the  margin  of  Jerome  Park  reservoir  at  205th  street. 


GEOLOGY  OK  THE  NEW   YORK  CITY  AQUEDUCT 


26V) 


2  East  side  of  Villa  avenue  north  of  Bedford  Park  boulevard. 

3  East  of  the  Concourse  between  198th  and  199th  streets. 

4  South  side  of  196th  street  both  east  and  west  of  the  Concourse. 
One  of  these  occurrences  was  known  to  the  geologists  of  the 

United  States  Geological  Survey  [New  York  City,  Eolio  No.  83]  but 
it  was  regarded  by  them  as  an  infold  of  the  Inwood.  An  examina- 
tion of  all  four  occurrences  will  convince  one  that  they  are  not 
infoldings.  In  at  least  two  cases  the  structure  accompanying  the 
beds  is  actually  anticlinal  instead  of  synclinal. 

These  occurrences  in  the  vicinity  of  Jero.ne  Park  reservoir  are 
essentially  the  same  as  those  disclosed  by  the  borings  of  the  Lower 
East  Side.  In  spite  of  its  thick  drift  cover  —  50  to  200  feet  — 
there  are  more  limestone  interbeds  known  there  than  in  any  other 
area  of  similar  size  in  tbe  region.  It  is  entirely  possible  that  a 
thorough  exploration  in  certain  other  belts  might  reveal  an  equally 
elaborate  development  elsewhere. 

The  substantiation  of  interbedded  limestones  as  a  prominent 
element  in  certain  fades  of  the  gneiss  series  and  their  association 
with  typical  silicious  gneiss  layers  with  transitional  relation  em- 
phasizes still  more  the  strictly  sedimentary  origin  of  at  least  som; 
portions  of  the  Fordham  series.  Other  observations  lead  to  the 
conclusion  that  they  are  the  oldest  members  of  the  series  and  that 
the  igneous  associates,  of  which  there  are  many,  are  all  younger 
intrusives. 

One  of  these  later  intrusives  is  the  Ravenswood  granodiorite 
which  cuts  into  the  eastern  margin  of  the  Lower  East  Side,  forms 
the  floor  of  the  present  East  river  channel  at  the  point  of  aqueduct 
crossing  and  continues  as  far  as  explorations  have  been  carried  into 
Brooklyn. 

Structural  detail  of  Lower  East  Side 

What  the  detailed  structure  of  the  Lower  East  Side  is,  it  is  im- 
possible to  say  at  the  present  stage  of  exploratory  development.  Its 
general  features  of  structure  are  fairly  clear.  The  Manhattan  schist, 
which  is  the  universal  floor  rock  of  the  central  and  western  parts 
of  Manhattan  island,  extends  only  a  short  distance  east  of  the 
Bowery.  The  Inwood  limestone  comes  to  the  surface  of  the  floor 
at  Christie  and  Forsyth  streets.  An  anticlinal  ridge  of  gneiss  comes 
up  at  Eldridge  and  Allen  streets.  Then  a  syncline  of  Inwood 
limestone  is  pinched  into  the  next  three  or  four  blocks  and  from 


2/0 


NEW  YORK  STATE  MUSEUM 


this  point  eastward  —  from  Norfolk  street  nearly  to  the  East  river — 
the  Fordham  gneiss  with  many  interbeds  of  limestone  forms  the 
rock  floor. 

As  much  of  this  detail  as  it  is  now  possible  to  classify  has  been 
included  in  the  accompanying  drawing,  plate  38,  in  which  special 
attention  has  been  given  to  the  interbedded  limestone  occurrences. 

In  view  of  the  fact  that  a  tunnel  is  finally  to  be  constructed 
through  this  section  which  will  cut  the  whole  series  of  formations 
and  structures  at  a  depth  probably  between  el.  -600  and  -700  feet, 
it  is  clear  that  much  greater  accuracy  of  geologic  interpretation 
is  soon  to  be  attainable  on  many  of  the  more  obscure  points.  Be- 
cause of  this  also  it  is  not  advisable  to  attempt  a  detailed  structural 
cross  section  at  the  present  time.  It  can  very  well  await  the  more 
complete  data  to  be  gathered  during  construction  of  the  tunnel. 


N  Y  Bute  Muuum  Bulletin  146 


PtaM  38 


¥■  /OO 
00 

-/oo 
-?oo 

-300 
-400 

-soo 


lVo,0v    Jo  °) 


,0, 


/rom  io 


7"A«  lower  pro/tic  line  it  intended  to 
A  /Af  limit  0/  rock  decoy  en  interpreted 
at    Both  prominent  depiemon 


nd.tote 


one  belon 


Sc/r/st-    S    ~    I  I 
A//#esfa*r-  /  -AS-  -  — 
G/re/ss  ■    Q    -    1  1 
/fecenrry  oyer  ?5/°  S/70h//7  ///i/j 
rfecore/y  i/mfer  "         "  " 
Ai?  rfttcorery  "  '• 


NOTf 


£orr/tyS  a/r yb/vyecfcJ />arv//e/ /b  s/>-/A-e 
o/r  ///re ye/mrr?  Jra/e  *<?fc 
/4ppn>x/mafe  foirr//??  of  s/rr*e  /s  /V ?SY 


300  Cily  0<  New  fork 

/ooo  BOARD  OF  WATER  SUPPLY 

CATSKILL  AQUEDUCT 

GEOLOGIC  DETAIL  OF  LOWER  EAST  SIDE 


OCTOBER  n.  1910 


CHAPTER  XX 


THE  GENERAL  QUESTION  OF  POSTGLACIAL  FAULTING 

WITH   ITS  BEARING  ON  THE  PERMANENCE  OF  ENGINEERING  STRUC- 
TURES. 

Evidences  of  postglacial  faulting  and  other  recent  movements 
have  of  late  attracted  a  good  deal  of  attention.  The  experience  of 
San  Francisco  in  the  exceptionally  disastrous  earthquake  and  lire, 
traceable  directly  to  earth  movements  of  the  nature  of  faulting 
which  dislocated  or  injured  the  water  conduits  rendering  them  tise- 
lcss,  is  fresh  in  the  minds  of  men  everywhere  who  have  public 
responsibilities  of  this  kind.  If  displacements  are  occurring  at 
the  present  time,  or  if  any  related  movements  are  continuing,  or  if 
there  is  evidence  of  recent  disturbances  of  this  sort  in  this  region, 
they  have  a  decidedly  important  bearing  upon  the  permanence  of 
all  engineering  structures  that  cross  them. 

Xo  undertaking  is  more  vitally  concerned  with  this  question  than 
the  Catskill  aqueduct.  Although  the  principal  factors  to  be  taken 
into  account  have  been  considered  in  other  connections  [see 
"Faults"  and  "Folds,"  pt  i]  a  unified  statement  may  encourage 
a  more  intelligent  understanding  of  the  bearing  of  these  structures 
in  southeastern  New  York  on  this  specific  question. 

The  region  included  in  this  discussion  extends  from  the  Catskill 
mountains  to  New  York  city.  It  will  be  convenient,  for  the  pur- 
poses of  this  argument,  to  divide  the  whole  area  into  three  districts 
whose  boundaries  are  determined  by  decided  differences  in  com- 
plexity of  geologic  history.  These  lines  necessarily  follow  closely 
the  boundaries  of  greater  stratigraphic  unconformities.  The 
youngest  groups  of  strata  have  suffered  only  such  changes  as  have 
accompanied  movements  of  later  geologic  periods.  But  before  they 
were  formed  the  underlying  groups  of  rocks  were  just  as  pro- 
foundly affected  by  earlier  disturbances.  In  this  region,  at  least, 
three  such  groups  of  large  importance  exist.  The  oldest  or  lowest 
has  been  affected  by  not  only  everything  that  has  influenced  the 
younger  strata  but  by  disturbances  of  a  still  earlier  time  which  verv 
much  increase  their  complexitv. 

On  this  basis  it  is  convenient  to  think  of  the  three  districts  as 
follows : 

A  Catskill  district.  Including  that  portion  of  the  region  west 
and  northwest  of  the  Shawangunk  mountains  and  marked  by  the 

271 


2~2  NEW  YORK  STATE  MUSEUM 

prevalence  of  Siluric  and  Devonic  strata,  i.  e.  all  strata  above  the 
Hudson  River  slates.  These  strata  have  been  affected  by  only  one 
great  mountain-making  movement  —  that  of  the  Appalachian  fold- 
ing, and  minor  disturbances  of  still  later  date. 

B  Hudson  river  district.  This  includes  that  portion  of  the 
region  lying  between  the  northern  border  of  the  Highlands  and  the 
Shawangunk  mountains.  It  is  marked  by  the  prevalence  of  Cam- 
bric and  Ordovicic  strata,  i.  e.  Hudson  River  slates,  associated  with 
Wappinger  limestone  and  Poughquag  quartzite  as  the  chief  bed 
rock.  These  strata  have  been  affected  not  only  by  the  Appalachian 
folding  but  also  by  a  still  earlier  one  —  that  of  the  Green  mountains 
and  the  Taconic  range.  They  were  folded  into  mountain  ranges 
and  worn  down  in  part  again  before  the  Siluric  and  Devonic  strata 
of  district  A  were  in  existence.  Therefore  as  a  structural  problem 
this  district  (B)  is  approximately  twice  as  complex  as  the  other. 

C  Highlands  district.  This  includes  all  of  the  region  com- 
monly known  as  the  Highlands  of  the  Hudson  as  well  as  the  rest 
of  the  area  south  of  the  Highlands  proper  to  New  York  city.  Its 
rocks  are  the  oldest  —  much  the  oldest.  They  had  been  folded  into 
mountain  structures  and  in  part  worn  down  before  any  of  the 
others  were  accumulated.  They  have  also  suffered  extensive 
igneous  intrusion  so  that  in  places  these  igneous  types  prevail. 
And  besides  they  have  been  metamorphosed  far  beyond  the  point 
of  any  other  group.  Xo  other  series  of  strata  has  been  so  pro- 
foundly affected.  They  form  the  lowest  group.  All  things  con- 
sidered this  district  should  be  structurally  three  times  as  compli- 
cated at  the  first  one  (A),  and  adding  the  igneous  and  metamorphic 
complexities,  it  is  probably  more  near  the  truth  to  consider  this 
Highland  district  four  or  five  times  more  complex. 

All  of  the  formations  from  the  oldest  to  the  Middle  Devonic  are 
involved.  For  the  specific  formations  and  their  succession  and  rela- 
tion the  reader  is  referred  to  that  discussion  in  part  I  [see  p.  29, 
et.  seq.]. 

Structural  features 

Except  the  most  westerly  part  of  the  region,  that  occupied  by  the 
Upper  Devonic  strata,  all  formations  are  compressed  into  folds. 
Many  of  the  smaller  folds,  especially  those  in  the  Catskill  district, 
are  still  complete.  The  easy  subdivision  of  strata  possible  in  this 
district  also  simplifies  the  problem  of  detecting  small  changes  of 
altitude.  Rut  for  the  most  part  the  larger  folds  have  been  beveled 
off  extensively  by  surface  erosion  so  that  only  the  truncated  limbs 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


273 


are  now  to  be  seen,  and  the  strata  therefore  appear  as  narrow  belts 
that  dip  steeply  into  the  ground.  This  is  more  marked  in  the 
Hudson  river  district  than  in  the  Catskill,  and  is  still  more  strikingly 
true  of  the  Highlands. 

There  are  evidently  at  least  three  different  epochs  of  folding  inter- 
rupting the  processes  of  sedimentation  and  followed  by  periods  of 
erosion  before  sedimentation  was  again  resumed.  These  breaks 
constitute  so  called  stratigraphic  unconformities  and  occupy  the 
relative  positions  indicated  in  the  foregoing  tabulated  scheme  [see 
pt  1]. 

In  each  epoch  of  folding  the  compressive  forces  accomplishing 
this  work  seem  to  have  acted  in  a  southeast-northwest  direction 
causing  successive  series  of  folds  with  a  northeast-southwest  trend. 
The  total  amount  of  crustal  shortening  accompanying  these  move- 
ments is  not  known,  but  that  it  must  be  many  miles  is  indicated  by 
the  fact  that  the  strata  of  the  older  series  of  formations  stand  pre- 
vailingly on  edge.  All  stages  between  small  amount  of  movement 
to  very  great  displacement  are  represented. 

Accompanying  the  folding  in  each  epoch  there  has  been  a  ten- 
dency to  rupture  and  displacement  of  the  "  fault  "  type.  There  are 
multitudes  of  them  varying  from  movements  of  too  little  amount  to 
be  regarded  in  a  broad  way  to  those  of  several  hundred  feet.  Most 
of  the  larger  and  more  persistent  ones  are  strike  faults  and  follow 
the  main  ridges  or  valleys,  sometimes  governing  the  location  of 
escarpments  or  gorges.  Dip  faults  crossing  the  formations  also 
occur  and  doubtless  have  guided  the  adjustments  of  many  tributary 
streams,  and  occasionally  portions  of  the  larger  water  courses.  The 
thrust  fault  is  most  common.  This  is  especially  true  of  the  larger 
ones  and  particularly  those  parallel  to  the  trend  of  the  other  struc- 
tural features. 

The  chief  effects  of  these  movements  may  be  summarized  as 
follows : 

1  Formations  are  cut  out  of  their  normal  order  and  nonadjacent 
ones  are  brought  in  contact. 

2  Cliffs  (escarpments)  and  sharp  gulches  are  more  common. 

3  Crush  zones  (belts  of  brecciated  material)  are  developed. 

4  The  crush  zones  give  an  additional  control  of  stream  adjust- 
ments. 

All  of  these  effects  are  common.  Many  of  those  faults  dating 
back  to  the  earlier  epochs  are  obscure  and  not  readily  located.  Many 
of  the  older  weaknesses  of  this  sort  have  been  healed  by  recry^talli- 


274 


NEW   YORK  STATE  MUSEUM 


zation  so  that  they  are  now  as  sound  as  any  other  portion  of  the 
rock.  A  good  deal  depends  upon  the  type  of  rock  and  the  conditions 
under  which  the  movement  took  place.  In  some  of  the  more  open 
ones,  circulating  water  has  seriously  affected  the  rock  and  in  places 
there  is  extensive  decay  even  in  the  harder  crystalline  formations. 

Age  of  the  faulting.  The  chief  epochs  of  folding  and  faulting 
are  those  of  the  mountain-making  movements  —  one  Precambric, 
another  Postordovicic,  and  still  another  Postcarbonic.  All  of  these 
ck'te  very  far  back  in  geologic  history,  and  since  the  last  of  these, 
nothing  akin  to  them  in  importance  has  been  felt  in  the  region. 

In  Posttriassic  times  however  there  was  small  faulting  south  of 
the  Highlands,  that  affected  the  areas  of  Triassic  rocks  of  New 
Jersey  and  Connecticut. 

Whether  or  not  there  continued  to  be  slight  movement  along  some 
of  the  older  lines  it  is  now  impossible  to  say.  It  is  at  least  clear  that 
all  of  the  great  movements  belong  to  very  ancient  time,  and  that  the 
last  period  of  geologic  time  as  we  know  it  for  this  region,  has  been 
one  of  comparative  stability.  The  chief  exception  is  evidently  con- 
nected with  the  continental  elevations  and  depressions  of  the 
glacial  epoch. 

Recent  movements.  The  effects  of  glaciation  make  it  possible 
to  determine  whether  or  not  there  has  been  further  movement  in 
postglacial  time.  Conditions  are  not  everywhere  favorable  enough 
to  detect  minute  changes,  but  where  they  do  obtain,  the  evidence  is 
capable  of  very  definite  interpretation.  The  essential  features  of 
these  conditions  are 

1  A  bed  rock  ledge  that  has  been  left  well  smoothed  by  glacial 
scouring. 

2  Protection  from  postglacial  destruction  so  that  the  original 
unevenness  as  left  by  the  glacial  smoothing  can  not  be  mistaken. 

If  on  such  a  ledge,  as  now  exposed,  there  are  steplike  offsets  or 
minute  escarpments  that  could  not  have  remained  had  they  been 
present  during  the  ice  action,  then  there  must  have  been  displace- 
ment to  this  extent,  since  the  original  smoothing  took  place. 

A  few  such  evidences  have  been  found  in  New  York  and  New 
England,  and  have  been  noted  in  geologic  reports.  W.  \Y.  Mather 
in  his  report  on  the  First  District  of  New  York  ( 1843)  pages  156-57, 
was  the  first.  The  data  as  now  known  may  be  found  in  the  last 
bulletin  of  Geologic  Papers  of  the  New  York  State  Survey  [see 
N.  Y.  Slite  Mus.  Bui.  107  (1907)  p.  5-28].    The  following  para- 


GEOLOGY  OF  THE  NEW  YORK  CITY  AQUEDUCT 


2/5 


graphs  are  intended  as  a  brief  summary  and  comment  on  the  facts 
as  there  given : 

Localities  where  some  postglacial  displacement  has  been 
detected. 

1  Copake,  X.  V.,  on  the  eastern  border  of  the  State  near  the 
southwest  corner  of  Massachusetts 

2  Rensselaer.  X.  Y. 

3  South  Troy.-X.  Y. 

4  Defreestville,  N.  Y.  (near  Troy) 

5  Pumpkin  Hollow,  X.  Y.  (near  Copake) 

6  Kilburn  Crag.  X.  H. 

7  Port  Kent.  X.  Y.  (uncertain) 

8  Attleboro.  Mass. 

In  addition  to  these  there  is  reference  to  similar  occurrences  at 
St  John,  XT.  B.  and  in  the  province  of  Quebec.  All  of  the  known 
localities  lie  a  considerable  distance  beyond,  north  and  northeast,  of 
the  Catskill  aqueduct  line. 

Causes  of  displacement.  In  southern  New  York  all  of  the 
cases  of  postglacial  faulting  yet  discovered  lie  in  the  area  of  slates 
belonging  to  the  Hudson  River  series.  Whether  the  belt  now  occu- 
pied by  this  formation  is  therefore  to  be  considered  the  most  un- 
stable zone,  or  whether  there  is  some  tendency  to  slight  readjust- 
ment inherent  in  the  slates  themselves  causing  these  movements,  is 
not  clear.  It  would  seem  consistent  with  known  recent  geologic 
history  to  connect  these  displacements  with  the  general  elevation 
and  subsidences  accompanying  and  following  the  glacial  occupation. 
It  is. perfectly  clear  that  the  whole  continental  border  in  this  region 
suffered  considerable  subsidence  during  glacial  time.  Also  the  ter- 
races and  deposits  along  the  Hudson-  prove  beyond  question  that 
during  the  ice  retreat,  at  the  very  close  of  the  glacial  occupation, 
the  land  surface  stood  from  So  to  150  feet  lower  than  now.  There- 
fore an  elevation  of  this  amount  has  occurred  in  postglacial  time, 
and  probably,  judging  from  the  condition  of  the  terraces  themselves, 
took  place  soon  after  the  glacial  ice  withdrew. 

The  stresses  and  inevitable  warpings  accompanying  these  mass 
movements  seem  to  be  sufficient  to  account  for  all  displacements 
known  to  be  of  this  age.  There  is  nothing  in  them  that  necessarily 
promises  a  renewal  of  mountain  folding.  But  it  appears  that  the 
movements  liave  almost  all  been  of  the  thrust  character  and  in  this 
respect  they  differ  not  at  all  from  the  commoner  type  of  the  region. 


2-6 


NEW  YORK  STATE  MUSEUM 


Amount  of  displacement.  The  greatest  throw  noted  on  any 
single  Postglacial  fault  in  eastern  New  York  is  given  by  Wood- 
worth  as  6  inches,  and  he  remarks  that  this  is  imperfectly  shown. 
Usually  the  displacement  is  distributed  over  a  zone  in  which  several 
small  faults  occur  instead  of  a  single  larger  one.  This  may  mean 
that  the  whole  disturbance  is  essentially  superficial. 

At  South  Troy  it  is  stated  that  a  total  displacement  of  12  inches 
is  thus  distributed  through  a  number  of  small  faults  within  a  dis- 
tance of  30  feet. 

At  Rensselaer  a  total  of  5  inches  is  given. 

At  Defreestville  a  total  of  13  inches  is  indicated  in  a  distance 
of  11.67  feet- 

At  Copake,  at  two  different  spots,  a  total  of  more  than  7  iuches 
was  measured  within  a  space  of  12  feet.  Woodworth  thinks  that 
the  total  displacement  for  the  locality  may  exceed  2  feet. 

At  Pumpkin  Hollow  a  total  of  17  inches  is  estimated. 

Conclusion.  If  such  rates  prevail  over  larger  areas  beneath 
the  drift,  it  is  clear  that  rather  profound  changes  would  be  indi- 
cated.   But  thus  far  there  is  no  indication  of  such  continuity. 

Likewise  if  it  were  certain  that  the  movements  are  now  in 
progress,  it  would  be  a  matter  of  greater  concern.  But  there  is 
no  direct  evidence  to  prove  it. 

Estimates  of  the  length  of  postglacial  time  differ  greatly.  The 
shortest  ones  worthy  of  consideration  range  from  about  5000  to 
10.000;  the  longest  run  above  100,000  years. 

Some  intermediate  value  is  probably  nearer  the  truth  —  say 
25,000  years. 

Adjusting  the  postglacial  faulting  problem  then  to  these,  time 
estimates  the  summary  of  it  all  would  be  as  follows :  Somewhere 
within  postglacial  time,  i.  e.  approximately  25,000  years,  move- 
ments of  strata  have  developed  at  a  few  places  in  eastern  New 
York  that  appear  as  small  faults  with  total  throw  in  each  locality 
varying  from  a  few  inches  to  perhaps  as  much  as  2  feet.  Whether 
the  movement  has  been  gradual  and  continuous  or  concentrated 
largely  into  some  small  portion  of  this  time  is  not  known.  Whether 
the  effects  are  extensive  or,  on  the  contrary,  very  local  and  super- 
ficial, is  likewise  unknown.  But  in  any  case  there  are  no  known 
instances  of  violent  and  large  displacements,  such  as  would  be 
likely  to  cause  great  damage  to  sound  structures,  in  this  region  in 
postglacial  time. 


INDEX 


Appalachian   mountain-folding.  63, 

66,  73- 

Aqueduct,  see  Catskill  aqueduct. 

Arden  point,  97,  104. 

Arden  point  line,  85. 

Artesian  flows,  142. 

Ashokan  dam,  construction  of,  13 ; 
elevation  of  reservoir,  17;  stone 
used  in  construction,  38;  geologi- 
cal features  involved  in  selection  of 
site  for,  109-16;  location  map,  113; 
Olive  Bridge  preferable  location, 
116;  to  be  finished  first,  1S3. 

Aspidocrinus  scutelliformis,  42. 

Athyris  spirifcroides,  38. 

Atrypa  reticularis,  40,  43. 
spinosa,  40. 

Atwood,  T.  C,  acknowledgments  to, 
7;  division  engineer,  237. 

Beaver  Kill,  no. 

Becraft  limestone,  42,  55,  126. 

Bensel,  John  A.,  member  of  Board 
of  Water  Supply,  13. 

Borkey,  Cliarles  P.,  consulting  geolo- 
gist, 6,  19,  75 ;  cited,  48. 

Binnewater  sandstone,  44,  55,  126, 
133.  134.  140;  porosity,  135. 

Bluestone,  character  and  quality, 
117-23;  economic  features,  119; 
petrography,  119-23. 

Borings  on  the  lower  east  side,  tabu- 
lations and  interpretations,  254-65. 

Breakneck  ridge,  85,  91-95,  100,  163 ; 
quality  and  condition  of  rock,  106- 

Brink,  Lawrence  C,  acknowledg- 
ments to,  6;  division  engineer,  21, 
140,  151. 

Bronx  valley,  geologic  cross  section, 
193- 

Brown,  Thomas  C,  employed  on 
Esopus  division,  125;  observation 
on  limestone  rocks.  140. 


Brush,  William  \\ ".,  acknowledg- 
ments to,  6;  division  engineer,  14, 
21,  215. 

Bryn  Mawr,  geologic  section  at,  205 ; 

comparison  with  shaft  13.  212. 
Bryn  Mawr  siphon.  201-8. 
Bull  mountain,  163. 

Calyx  drill,  26. 

Cambric  quartzite,  102. 

Cambro-Ordovicic  formations,  45-46, 
56,  63. 

Carson.  J.  P.,  cited,  209. 

Cat  Hill  gneissoid  granite,  52,  57. 

Catskill  aqueduct,  water  supply  pro- 
ject, 9-16;  generalities  of  construc- 
tion, 14-15;  estimation  of  cost,  15; 
present  plans  for,  15;  time  for 
completion.  15 ;  problems  en- 
countered in  the  project.  17-24; 
gathering  data,  21-23  >  relative  val- 
ues of  different  sources  of  infor- 
mation and  stages  of  development, 
25-28  ;  geologic  problems.  75-276 ; 
general  position  of  aqueduct  line, 
77-80 :  location  map,  80. 

Catskill  creek,  II. 

Catskill  district,  general  geology,  29- 
74:  of  simple  structure,  31;  post- 
glacial faulting,  271-72. 

Catskill  formation,  37,  55,  63. 

Catskill  Monadnock  group,  73. 

(  ai  skill  supply,  area  in  square  miles, 
11:  daily  supply  in  gallons,  11; 
estimated  daily  supply,  11;  esti- 
mated cost,  1 1  :  storage  in  gallons, 
11  ;  part  of  supply  available  by 
79/?,  15. 

Catskill  system,  parts  of.  11—14;  con- 
struction of  certain  parts  in  ad- 
vance of  the  rest,  13. 

Catskill  watersheds  and  aqueduct, 
map,  12. 

Caves,  137. 


-'77 


278 


NEW  YORK  STATE  MUSEUM 


Cedar  Cliff,  103. 
Cement  beds,  44. 
Cenozoic  time,  64. 

Chadwick,   Charles   N.,  member  of 
Board  of  Water  Supply,  13. 

Chonetes  coronatus,  38. 
mucronatus,  38. 

Chop  drill,  26. 

Clapp,    Sidney,    assistant  engineer, 
109. 

Clark,  cited,  44. 

Clays,  in. 

Coastal  plain,  73. 

Cobleskill  beds,  4^,  55,  126. 

Coeymans  limestone,  43,  55,  126,  133. 

Cold  Spring,  85,  163. 

Conglomerates,  better  quality  of  wall 
than  limestones,  140. 

Continental  elevation,  67-69. 

Cortlandt  series  of  gabbro-diorites, 
52,  53.  57- 

Coxing  kill,  127,  128. 

Coxing  kill  section,  135-36;  struc- 
tural geologic  detail,  136. 

Cretaceous  deposits,  36-37,  54. 

Cretaceous  penpplain,  67. 

Cronomer  hill,  154. 

Crosby,  W.  O.,  consulting  geologist, 
6,  T9>  75 ,'  cited.  36. 

Cross  sections,  Rondout  valley,  140. 

Croton  aqueduct,  study  of  shaft  13 
and  vicinity,  209-14;  comparison 
of  Bryn  Mawr  and  shaft  13,  212- 
14;  map  showing  location,  239. 

Croton  lake  crossing,  183-89. 

Croton  river,  average  daily  flow,  9. 

Croton  water,  average  daily'  consump- 
tion, 9. 

Crows  Nest,  100,  163. 

Crystallines,  south  of  the  Highlands, 
47.  56;  older,  57. 

Dalmanella  testudinaria,  46. 
Dalmanites  selenurus,  40. 
Dana.  J.  D„  cited.  48;  mentioned,  46. 
Danskammer  crossing,  85,  97,  103. 
Darton,  mentioned,  46. 
Davis,  Carlton  E.,  acknowledgments 
to,  6;  department  engineer,  13. 


I  Davis,  J.  L.,  tests  of  Kensico  rocks, 
199. 

Delancey  and  Clinton  street  section, 
structural  geology,  253-66. 

Delivery  conduits,  geological  condi- 
tions affecting  the  location  of  con- 
duits, 215-29. 

Devonic  strata,  37-43,  55. 

Diabase,  37,  240. 

Diamond  drill,  26. 

Dikes,  106;  pegmatite,  52. 

Dinnan  quarry,  198. 

Division  engineers,  responsibility  of, 
21. 

Drift,  kinds  of,  33-36;  glacial,  32-36, 

54,  100,  202. 
Dwight,  mentioned,  46. 

East  river  crossing,  233,  238. 
East  river  section,  250-53 ;  structure, 
233. 

Engineers,  division,  responsibility  of, 
21. 

Esopus  creek,  11,  69,  77,  112,  128. 

Esopus  division  of  Northern  Aque- 
duct Department,  125. 

Esopus  shale,  40,  55,  126,  140;  thick- 
ness, 133. 

Esopus  valley,  geologic  cross  section, 
78. 

Esopus  watershed,  development  of, 
13.  15. 

Exploration  zones,  special,  237-70. 
Fault  zones,  266. 

Faults,  60-62.  135,  163:  postglacial, 

general  question  of.  271-76. 
Favosites  helderbergia,  43. 
Ferris  quarry,  198. 
Firth  Cliffe,  153. 

Flinn,  Alfred  D..  acknowledgments 
to,  6;  department  engineer,  13,  215. 
Folds,  59-60,  272. 

Fordham  gneiss,  47,  52,  57,  62,  185, 
191,  192,  202,  206,  217,  218,  219, 
220,  221,  225,  226,  232,  233,  234, 
237.  238.  255,  257,  258,  260,  261, 
262,  264,  265,  266,  268. 


INDEX  TO  GEOLOGY  OE  THE 


NEW  YORK  CITY  AQUEDUCT 


Formations,  summary  of,  54-57. 

Foundry  brook,  27 ;  rock  condition 
at,  163-69. 

Foundry  brook  valley,  structural  de- 
tail, 165. 


Garden  quarry,  198,  199. 

Geographic  features,  30-31. 

Geographic  history,  65-74. 

Geologic  conditions  affecting  the 
Hudson  river  crossing,  97-107. 

Geologic  knowledge,  practical  appli- 
cation to  engineering  plans,  19. 

Geologic  problems  of  the  aqueduct, 
75-276. 

Geology  of  region,  29-74 !  summary 
of  formations,  54-57;  outline  of 
history,  62-65  >  local  summary,  265- 
66. 

Glacial  drift,  32-36,  54,  100,  202. 

Glacial  period.  64,  71. 

Gneisses,    176;    dioritic,    198,  199; 

Grenville    series,    50-52,    57.  See 

also  Highland  gneiss. 
Grabau,  A.  \Y..  cited,  37,  44. 
Granites,    99,    100,    106 ;  gneissoid, 

198;  of  the  new  Ferris  quarry,  198. 
Grassy  Sprain  valley,  203. 
Gravel,  no. 
Gravel  hillocks,  no. 
Gravel  streaks,  11 1-12. 
Grenville  series,  50-52,  57,  62. 


Hamilton  shales,  38,  55,  78,  no,  126, 
140. 

Harbor  hill  moraine,  35. 

Harlem  river  crossing,  237,  238-44 ; 

map  showing  plan  of  exploratory 

borings,  239. 
Hartnagel,  cited,  44;  mentioned,  154. 
Healey,  John  R.,  acknowledgments  to, 

6 ;  exploratory  work  by,  237. 
Henry  street,  interpretation  of  hole 

No.  207  on,  261-65. 
Hester  street,  interpretation  of  hole 

No.  202  on,  259-61. 
High  Falls,  125,  127,  133. 


High  Falls  shale,  44,  55.  126,  133.  '34. 

135 ;  porosity,  135. 
Highlands,    30-31,    73,    81;  crystal 

lines  south  of,  47,  56;  postglacial 

faulting,  272. 
Highlands-  gneiss,  50,  57,  99,  102,  154, 

163.    See  also  Fordham  gneiss. 
Highlands  group,  crossings,  97,  103- 

4,  105 ;  more  defensible  as  a  route 

for  the  aqueduct  line,  103. 
Hill  View  reservoir,  215;  elevation, 

17- 

Hipparionyx  proximus,  41. 

Hobbs,   mentioned,  95. 

Hogan,  Thomas  H.,  assistant  di- 
.  vision  engineer,  125. 

Hornblendic  gneiss,  263. 

Hudson  river,  69;  water  to  be  used 
for  lire  protection,  10;  wash  bor- 
ings, 26;  depth  of  buried  channel, 
89;  submarine  channel,  90-91; 
Storm  King-Breakneck  mountain 
profile,  91-95;  origin  of  the  present 
course,  95-96;  crossing,  geological 
conditions  affecting,  97-107 ;  out- 
line map  showing  possible  cross- 
ings, 98;  difference  of  structure 
in  crossings,  104;  postglacial  fault- 
ing of  district,  272. 

Hudson  river  canyon,  81-96;  points 
of  exploration,  83-88;  comparative 
sections  at  Peggs  point  and  Storm 
King,  92:  study  of  profile,  94. 

Hudson  River  slates,  46,  56,  83,  100, 
102,  103,  126,  135,  137,  140,  149. 
153,  154,  272. 

1  Iudson  schist,  201. 

Hurley,  127. 

Idlewild,  154. 
Igneous  types,  52-54. 
Imperviousness  and  insolubility,  138- 
39- 

Inwood  limestone,  47,  49-50,  56,  172, 
185,  191,  192,  201,  202,  210,  212, 
217,  218,  219,  220,  221,  226,  232, 
237,  238,  240,  242,  243.  245,  246, 
249,  254,  255,  256,  261,  262,  255, 
268,  269. 


280 


NEW  YOKK  STATE  MUSEUM 


Ithaca  beds,  38,  55. 

Jameco  gravels,  36. 

Jerome  Park  reservoir,  interbedded 
development  of  limestones  in  vicin- 
ity of,  268. 

Jura-Trias  formations,  37,  54. 

Kemp,  James  F.,  acknowledgments 
to,  6;  consulting  geologist,  6,  19, 
75;  cited,  81,  232. 

Kensico  dam  site,  geology  of,  191- 
94- 

Kensico  quarries,  stone  of,  195-200; 

additional  tests,  199. 
Kensico   reservoir,   to  be  enlarged, 

13. 

Kripplebush,  127. 
Kripplebush  section,  129-31. 

Laminated  sand  and  clay,  111. 

Laminated  till,  HO. 

Langthorn,  J.  S.,  acknowledgments 
to,  7;  division  engineer,  21,  109. 

Leperditia  alta,  44. 

Leptaena  rhomboidalis,  40,  42. 

Leptocoelia  acutiplicata,  40. 

Leptostrophia  inagnifica,  41. 
perplana,  40. 

Liberty  ville,  149,  150. 

Limestones,  99,  100,  176;  resistance 
to  solution,  139;  analysis  of,  139; 
of  Sprout  Brook  valley,  171  ;  in- 
terbedded, older  than  the  Inwood, 
266-70. 

Liorhynchus  mysia,  38. 

Little  Stony  point,  85,  97,  104. 

Location  map,  80. 

Long  Island,  Cretaceous  and  Ter- 
tiary strata,  32;  glacial  deposits, 
35-36. 

Lower  East  Side  zone,  238. 
Lowerre  quartzite,  47,  50,  56. 

McCarthy,  C.  H  ,  boring  equipment 
owned  and  operated  by,  141. 

Manhattan  schist.  47.  48-49,  56,  171, 
183.   186,  191,  192,  201,  217,  218. 


219,  220,  221,  225,  226,  233,  234, 
237.  238,  240,  241,  242,  243,  244, 
245,  246,  247,  248,  249,  254,  257, 
261,  265,  268,  269. 
Manhattanville  cross  valley,  237,  244- 
45- 

Manlius   limestone,   43-44,   55,  126, 

133,  U34- 
Marcellus  shales,  38,  55,  126. 
Matawan  beds,  37,  54. 
Mather,  W.  W.,  cited,  268,  274. 
Merrill,  cited,  48. 

Merriman,  Thaddeus,  acknowledg- 
ments to,  6;  assistant  chief  engi- 
neer, 13,  109. 

Mesozoic  time,  64. 

Miocene  deposits,  36. 

Miocene  fluffy  sand,  54. 

Moodna  creek,  103-4;  wash  borings, 
26;  course  of,  155-59. 

Moodna  valley,  ancient,  153-62;  sta- 
tistics, 160. 

Morningside  to  Central  Park  sec- 
tion, 245-50. 

Mountain-forming  movements,  59- 
60. 


New  Ferris  quarry,  granite,  198. 

New  Hamburg,  81. 

New  Hamburg  group,  crossings,  97, 

102-3,  105. 
New  Hamburg  line,  83-85. 
New  Paltz,  149. 

New  Scotland  shaly  limestone,  42, 
55.  126,  133. 

New  York-Wescchester  district.  30. 

New  York  city,  gorge  at,  91 ;  sec- 
tions of  gorge  at  32d  street,  92; 
geological  conditions  affecting  the 
location  of  delivery  conduits,  215- 
29;  areal  and  structural  geology 
south  of  59th  street.  231-36;  struc- 
tural geology  of  the  lower  East 
side,  253-66. 

Newark  series.  37.  54. 

Ncwburgh,  154. 

Northern  aqueduct  to  be  finished 
first,  183. 


INDEX  TO  GEOLOGY  OF  THE 
Oil-well  rig,  26. 

Olive  Bridge,  110;  site,  112-14;  pref- 
erable location  for  the  proposed 
Ashokan  dam,  116. 

Oneonta  formation,  38,  55,  119. 

Onondaga  limestone,  39-40,  55,  78, 
126,  127,  129. 

Oriskany  beds,  40,  55,  126. 

Orthothetes  woolworthanus,  42. 


Pagenstechers  gorge,  154.  155,  159. 

Paleozoic  time,  63. 

Palisade  diabase,  37,  54. 

Pebble  beds,  m-12. 

Peekskill  creek,  172. 

Peekskill  creek  valley,  structure  of, 

175-82;  geologic  cross  section  and 

detail  of  borings,  180. 
Peekskill  granite,  52,  53,  57. 
Peggs  point,  borings,  89;  gorge  at, 

91,  92. 

Peggs  point,  crossing,  83,  97,  103. 

Pegmatite,  53,  57,  185;  dikes,  52. 

Peneplain,  Cretaceous,  67. 

Pennsylvania   borings   opposite  33d 
street,  New  York  city,  89. 

Phyllite,  175-76,  181. 

Physiography,  30-31.  65-74. 

Piedmont  belt,  73. 

Platyceras  dumosum,  40. 
nodosum,  41. 

Pleistocene  glaciation,  71. 

Pliocene  deposits,  36,  54. 

Pompey's  cave,  137-38. 

Porosity  tests,  142-47. 

Porosity  of  Kensico  rocks,  199. 

Port  Ewen  beds,  40,  42,  55,  126. 

Postglacial    faulting,    general  ques- 
tion of,  271-76. 

Poughquag  quartzite,  47,  56,  100,  172, 
176,  181,  272. 

Pressure  tests,  27. 

Pumping  experiments,  142-47. 


Quartz,  202. 

Quartzite  beds,  99,  100,  176. 
Quaternary  deposits,  32-36,  54. 


NEW  YORK  CITY  AQUEDUCT  281 

Raritan  formation,  37,  54. 
Ravenswood  granodiorite,  52>  53.  57- 

217,  220,  221,  226,  233,  252,  265, 

269. 

Rhipidomella  oblata,  42. 

Ridgway,  Robert,  acknowledgments 
to,  6;  department  engineer,  13. 

Rondout  cement,  44. 

Rondout  creek,  n,  69. 

Rondout  creek  section,  131-35. 

Rondout  siphon  statistics,  141-42. 

Rondout  valley  section,  125-47;  en- 
gineering problems,  17-19 ;  geology, 
31  ;  special  features,  137-40;  analy- 
sis of  limestones,  139;  cross  sec- 
tions, 140. 

Ronkonkoma  moraine,  35. 

Rosendale  cement,  44,  45. 

Rosendale  limestone,  126. 

St  Nicholas  Park,  246. 

Sanborn.  James  F.,  acknowledg- 
ments to,  6;  division  engineer,  21, 
125,  149- 

Sand,  110,  in. 

Sandstones,  100. 

Saw  Mill  valley,  209. 

Schistose  beds,  99. 

Schoharie  creek,  11. 

Schoharie  shale,  40,  55. 

Sedimentation  structures,  58. 

Shales,  better  quality  of  wall  than 
limestones,  140. 

Shaw,  Charles  A.,  member  of  Board 
of  Water  Supply,  13. 

Shawangunk  conglomerate,  45,  55, 
63,  126,  127,  133,  135,  136,  149; 
thickness,  136;  overthrust,  137. 

Shawangunk  mountains,  31,  127. 

Sherburne  beds,  38,  55,  109,  119. 

Shot  drill,  26. 

Sicberell'a  galeata,  43 ;  figures,  43. 

pseudogaleata,  42;  figures,  42. 
Siluric  strata,  43-45,  55. 
Sing  Sing  marble,  201. 
Siphon  line,  total  borings  on,  141. 
Skunnetnunk  mountain,  153,  154. 


U$2 


NEW  YORK  STATE  MUSEUM 


Smith,  J.  Waldo,  credit  due,  5;  chief 

engineer,  13. 
Smith,  Merritt  II.,  acknowledgments 

to,  6;  deputy  chief  engineer,  13; 

department  engineer,  14,  183. 
Smith,  Wilson  F.,  acknowledgments 

to,  7;  division  engineer,  21,  191. 
Snake  hill,  153. 
Solubility,  question  of,  138. 
Southern  aqueduct,  general  location 

map,  184;  terminus,  215. 
Southern  aqueduct  department,  183. 
Spear,  Walter  E.,  acknowledgments 

to,  6;  department  engineer,  14,  215. 
Special  exploration  zones,  237-70. 
Spencer,  J.  W.,  cited,  90. 
Spirifer  arenosus,  41 ;  figures,  41. 

concinnus,  42. 

macropleura,  42 ;  figures,  43. 
mucronatus,  38;  figures,  39. 
murchisoni,  41. 
perlamellosus,  42. 

Sprague  &  Henwood,  boring  equip- 
ment owned  and  operated  by,  141. 

Sprain  brook,  203. 

Springtown,  149,  150. 

Sproul,  A.  A.,  acknowledgments  to, 
6;  division  engi-uer,  21,  172,  175. 

Sprout  brook,  175,  177;  geology,  171- 
74;  geologic  cross  section,  173. 

Staten  Island.  Cretaceous  and  Ter- 
tiary strata,  32. 

Stockbridge  dolomite,  201,  212. 

Stony  Point,  177. 

Storm  King  crossing,  85,  91-95,  97, 

104,  105;  trial  profile,  94. 
Storm  King  gorge,  89,  92,  156. 
Storm  King  granite,  52,  57,  100,  104; 

quality    and   condition    of  rock, 

106-7. 

Storm   King  mountain,   fault  along 

the  southeast  face  of,  163. 
Stratigraphy,  31-57- 
Strophalosia  truncata,  38. 
Stropheodonta  becki,  42. 
Strophonella  headleyana,  42. 
Strophostylus  cxpansus,  41. 
Structural  features,  58-62. 
Stviiolina  fissurella,  38. 


Surface  configuration,  history  of,  66. 
Swift,  William  E.,  acknowledgments 
to,  6;  division  engineer,  21,  83,  163. 

Taonurus  caudagalli,  40. 
Tertiary  deposits,  36-37,  54. 
Tertiary    incomplete  peneplanation, 

69-70. 

Tertiary  reelevation,  70-71. 

Thirlmere  aqueduct  of  the  Man- 
chester, England,  Waterworks,  138. 

Thirlmere  limestone,  average  of  five 
analyses,  139. 

Tibbit  brook  valley,  203. 

Till,  no. 

Tompkins  Cove,  177. 

Tongore  site,  1 14-16;  plan  and  geo- 
logic section,  114;  detail  of  drift 
character,  115. 

Topographic  features,  30-31. 

Tuckahoe  marble,  201. 

Tuff  crossing,  83. 

Uncinulus  campbellanus,  42. 
Unconformities,  58-59. 

Valhalla,  191. 

Van  Ingen,  cited,  44. 

Veatch,  cited,  35,  36. 

Wallkill  river,  69,  128. 

Wallkill  valley  section,  149-51 ;  drift 

conditions,  25. 
Wallkill-Newburgh  district,  31. 
Wappinger  limestone,  46,  56,  83,  100, 

102,  154,  172,  176,  181,  272. 

Wash  rig,  25,  81. 
Water,  increase  in  consumption,  9; 

reports    on    available   sources  of 

supply,  10.    See  also  Catskill  sup- 

ply. 

Water  Supply  Board,  staff,  acknowl- 
edgments to,  7;  members,  13;  de- 
partments, 13-14. 

Wegman,  cited,  209. 

West  Hurley,  77,  78. 


INDEX  TO  GEOLOGY  OF  THE  NEW   YORK  CITY   AQUEDUCT  28$ 


"West  Point  location,  104. 
West  Shokan,  til. 

White,  Lazarus,  acknowledgments 
to,  6;  division  engineer,  21,  125, 
142. 

AYilbur  limestone,  44. 
Winsor,  Frank  E.,  acknowledgments 
to,  6;  department  engineer,  14,  183. 


Woodlawn,  cited,  209. 
Woodworth.  mentioned,  276. 

Yonkers  gneiss,  igr,  106,  197,  198, 
202,  217,  218,  219,  220,  221,  225, 
226;  of  superior  durability,  200. 

Zaphrentis  prolifica,  40. 


MAP  SHOWINC 

CEOLOCIC  FORMATIONS 

ALONG  THE 

PROPOSED  LINE^FOR  DISTRIBUTION  COND 


Manhattan  Schist 


Inusood  Limestone 

tZZ] 

Fvrdham  Gneiss 
Yorkers  Gneiss 


Rauensufood  Granodiorit' 

G5  '1  \ 

Outcrops  of  Rock  i 
Faults  {cross  faults)  \ 

1 

Orxgmal  Lines,  A-B-C^D 
New  Lines rp-G-H-f  J 


M 


- 


as 


4 


